Dr. Jeff Masters' WunderBlog

Jimena nears Category 5 status; 94L getting sheared

By: JeffMasters, 9:02 PM GMT on August 31, 2009

The most powerful hurricane anywhere on the planet so far this year is Hurricane Jimena, according to data from this afternoon's hurricane hunter mission. Jimena's 155 mph winds beat out the South Pacific's Tropical Cyclone Hamish (150 mph winds) as the most powerful tropical cyclones so far this year. The Hurricane Hunters have completed their mission into Jimena, so we will have to rely on satellite estimates of Jimena's strength until Tuesday afternoon's hurricane hunter flight to see if the storm intensifies to the 160 mph threshold needed for it to become a Category 5 hurricane.

Jimena is expected to make landfall Tuesday night or Wednesday morning along the Mexico's Baja Peninsula. The hurricane is in an environment with low wind shear, 5 - 10 knots, and warm Sea Surface Temperatures (SSTs), 30°C. Shear is expected to remain low, and SSTs will decline to 28°C with a corresponding decree in total oceanic heat content between now and landfall, and these conditions should mean that Jimena will be a Category 3 or 4 hurricane at landfall. The computer models have come into better agreement with their latest 12Z runs, giving confidence that a landfall north of Cabo San Lucas will occur, and that town is now outside of the NHC cone of uncertainty. Cabo San Lucas has a 13% chance of receiving hurricane force winds, according to NHC's wind probability product. Serious flooding due to heavy rains will occur across all of the southern Baja Tuesday and Wednesday. Jimena is of similar intensity and is following a similar track to Hurricane Juliette of 2001, which brought 17.7" of rain to Cabo San Lucas. Juliette killed 7 people and caused $20 million in damage to Mexico, mostly due to flash flooding and mudslides from the heavy rains.

After Jimena makes initial landfall on Baja, it will cross over the Gulf of California and make landfall on Mainland Mexico. Depending upon how up along the coast this second landfall occurs, Arizona may receive moisture from Jimena late this week that will be capable of causing flooding rains.


Figure 1. Visible satellite image of Hurricane Jimena at 4:35 pm EDT on Monday, 8/31/09.

Invest 94L getting sheared
Tropical wave (94L) about 450 miles east of the central Lesser Antilles Islands, continues to be a threat to develop into a tropical depression. However, recent satellite wind shear analyses by NOAA/Colorado State University show that wind shear has increased throughout the day over 94L, and is now near 20 knots, which is marginal for development. Satellite shear analyses from the University of Wisconsin show about 10 - 20 knots of shear, and it is apparent from satellite imagery that shear is causing some disruption to the west side of 94L. Visible satellite imagery show that 94L does not have a surface circulation center, and the storm has not significantly increased in organization this afternoon.

The forecast for 94L
Wind shear as diagnosed by the SHIPS model currently shows a low amount of shear, 5 - 10 knots, and this is not correct, so this model's forecast of low shear over 94L for the next five days cannot be trusted. The latest set of 12Z model runs are more restrained in developing 94L, and are depicting higher levels of shear will be affecting 94L over the next three days. SSTs will be warm, in the 28 - 29°C range, and dry air should have only a minor inhibiting effect, but the high shear may delay 94L from developing into a tropical depression for several days. The HWRF makes 94L into a strong tropical storm 5 days from now, but the GFDL and GFS models do not develop 94L at all. The ECMWF and NOGAPS models predict that development of 94L will not occur until five or so days from now, in a region between the Bahama Islands and Bermuda. NHC continues to give 94L a high (greater than 50% chance) of developing into a tropical depression by Wednesday. This probability may need to get scaled back to moderate (30 - 50% chance) if the high shear continues into tomorrow.

My next post will be Tuesday morning.

Jeff Masters

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Hurricane warnings for Baja; 94L forming spiral bands

By: JeffMasters, 3:36 PM GMT on August 31, 2009

Hurricane warnings are in effect for the southern tip of Mexico's Baja Peninsula, where powerful Hurricane Jimena is expected to make landfall Tuesday night or Wednesday morning. The hurricane is in an environment with low wind shear, 5 - 10 knots, and warm Sea Surface Temperatures (SSTs), 30°C. Shear is expected to remain low, and SSTs will decline to 28°C with a corresponding decrease in total oceanic heat content between now and landfall, and these conditions should mean that Jimena will be a Category 3 or 4 hurricane at landfall. Outer rain-bands of the hurricane will be appearing on Los Cabos radar soon, though the Mexican Weather Service web site has been hard to reach today. The computer models are split, with one camp calling for a landfall in southern Baja, and the other camp calling for landfall farther north near central Baja. The official NHC forecast splits the difference between these two solutions, and landfall could occur anywhere along a long stretch of the Baja coast. At this point, the UKMET model's solution taking Jimena westward out to sea is being discounted, since it is an outlier.

After Jimena makes initial landfall on Baja, it will cross over the Gulf of California and make landfall on Mainland Mexico. Depending upon how up along the coast this second landfall occurs, Arizona may receive moisture from Jimena late this week that will be capable of causing flooding rains.


Figure 1. Image of Hurricane Jimena taken by NASA'a MODIS instrument at 2020 UTC Sunday, 8/30/09.

Invest 94L
The well-organized tropical wave (94L) near 14.5N, 52W, about 500 miles east of the central Lesser Antilles Islands, continues to be a threat to develop into a tropical depression. Visible satellite imagery and this morning's QuikSCAT pass do not show a surface circulation yet, though 94L does have a large envelope of moisture and some modest heavy thunderstorm activity. QuikSCAT noted winds up to 30 mph. Water vapor satellite loops show that 94L has moistened the region surrounding it considerably, and dry air from the Saharan Air Layer is not a major impediment to development. Wind shear is a low 5 - 10 knots, and the ocean temperature are a moderately warm 28°C. Visible satellite loops over the past two hours show low-level spiral bands developing on 94L's northeast side, and I give a 70% chance the Hurricane Hunters will find a tropical depression or tropical storm on Tuesday when they investigate 94L.


Figure 2. Morning satellite image of 94L, showing low-level spiral bands developing on the northeast side.

The center of 94L probably passed over Buoy 41040, located at 14.5N, 53W over the past hour. Winds blew northeasterly early this morning, then went calm, then shifted to southerly late this morning. The winds were less than 10 knots during the center passage, so the circulation of 94L is not yet well-defined. The pressure fell significantly as 94L moved over the buoy (seen only after one removes the wiggles due to daily atmospheric tide effect present in the tropics). 94L will appear on Martinique radar on Tuesday.

The forecast for 94L
Shear will remain low, 5 -10 knots, over the next 5 days, SSTs will be warm, in the 28 - 29°C range, and dry air should have only a minor inhibiting effect, so I can't see anything that will prevent 94L from developing into a tropical depression over the next 1 - 2 days. The HWRF model develops 94L into a hurricane 4 days from now, as does the SHIPS intensity model, but other models, such as the GFDL, ECMWF, and GFS, do not develop 94L at all.

Model solutions for the track of 94L are divergent. Water vapor satellite loops show two upper-level lows to the north and northwest of 94L that are pulling the storm to the west-northwest, and 94L's motion is expected to range between the west-northwest and northwest over the next three days. By Tuesday, 94L will slow down from its current 15 mph forward speed to about 10 mph. Most of the models predict that the steering influence of the upper-level lows will pull 94L far enough north that the storm will miss the Lesser Antilles, with a closest approach occurring Wednesday and Thursday. However, the ECMWF and HWRF models have 94L passing within 200 miles of the islands, and the northern Lesser Antilles may experience tropical storm conditions on Thursday.

At the longer ranges, the fate of 94L is highly uncertain. The Canadian model turns 94L to the north near Bermuda, then out to sea, while the NOGAPS model foresees a threat to the U.S. East Coast early next week. Both of these solutions are believable. The Hurricane Hunters are scheduled to make their first flight into 94L on Tuesday afternoon at 2pm EDT.

Elsewhere in the Atlantic
The remains of an old cold front are bringing cloudiness and showers to the northern Gulf of Mexico and waters offshore North and South Carolina. The GFS and NAM models indicate an area of low pressure may develop along this old front near the Florida Panhandle or off the coast of North Carolina by Thursday. However, such a low may be extratropical and not tropical.

My next post will be between 3 - 5pm this afternoon.

Jeff Masters

Hurricane

Updated: 5:18 PM GMT on August 19, 2011

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Category 4 Hurricane Jimena Approaches Baja California

By: JeffMasters, 7:50 AM GMT on August 31, 2009

Hi, this is Dr. Rob Carver filling in for Dr. Masters. Hurricane Jimena is the main story for the North American tropics with satellite-estimated winds of 145 mph. This makes Jimena a Category 4 storm on the Saffir-Simpson scale. However, later today, we will actually have measured winds for this storm. The Plan of the Day indicates that an Air Force reconnaissance mission to Jimena has been scheduled for the middle of the day Aug. 31. Hurricane watches have been posted for the southern Baja peninsula.

Jimena continues to have an impressive presentation on satellite with a compact core and cold cloud tops. Late in the afternoon, the TRMM satellite flew directly over Jimena, and part of the central core was in the field of view of TRMM's radar.


Fig. 1 TRMM overpass at 2355Z Aug 30 showing surface rain fall rates derived from microwave radiometry and Doppler radar. Image courtesy of NRL

This shows that Jimena has a a very tight eyewall, consistent with its intensity, with a possible eyewall replacement cycle starting soon. If the cycle starts, Jimena will likely weaken some, but this is still a dangerous storm. The AF reconnaissance mission will provide NHC's forecasts a good deal of information about the storm and improve the computer forecasts (There really isn't much in the way of measurements out there.) It is also important to note that as Jimena moves northward, it will move over cooler water, which is not favorable for maintaining its intensity.

The current track forecast calls for Jimena to make landfall on the western coast of Baja southwest of La Paz. Given the angle between Jimena's track and Baja's coastline, there is a wide spread of possible landfall locations. People living in this area need to be making hurricane preparations now.

Invest 94L Invest 94L is still out there, moving westwards to the Lower Antilles. People in the Lesser Antilles still need to keep an eye on this storm, as tropical storm formation is possible with this storm. This is still considerable uncertainty on the track of this feature as different models have different initializations of the storm. However, the Plan of the Day indicates that reconnaissance missions into Invest 94L could start as soon as Tuesday, September 1, which would give everybody more data to understand what's going on.

Hurricane

Updated: 5:20 PM GMT on August 19, 2011

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Invest 94L and the Lesser Antilles

By: JeffMasters, 9:51 AM GMT on August 30, 2009

Hi, this is Dr. Rob Carver filling in for Dr. Masters. The convection associated with Invest 94L has intensified, prompting the NHC to issue an early Tropical Weather Outlook to update the situation. Essentially, people in the Lesser Antilles need to start paying attention to the seas and skies. Looking at the computer track models, the set of model solutions are fairly similar to the Saturday model runs. A set of dynamical models (GFDL, HWRF, NOGAPS) runs to the NE of the Antilles, while the simpler models (LBAR, BAM family) are more easterly and take 94L over the Antilles. I do not ascribe any significance to the 00Z GFS track following the simpler models over the dynamical models. From my examination of the 00Z GFS run, it looks like the simulated 94L weakens too much, so the automatic vortex tracker picks up another vorticity feature. As for the future of 94L, there is a large area of large wind shear to it's north. It will need to keep it's track at WNW-W for intensification.

Update at 6AM EDT
I would be remiss if I didn't mention the major hurricane in the Eastern Pacific, Jimena At this time, Jimena has a satellite-estimated maximum wind speed of 115 mph, making it a Category 3 storm on the Saffir-Simpson scale. Jimena has an impressive presentation in satellite imagery with a large area of cold anvil cirrus in the center of the storm. The following microwave imagery indicate the formation of a relatively potent central eyewall, which is responsible for intensification.


Fig. 1 SSMI 85 GHz PCT at 0244Z Aug 30. Image courtesy of NRL.


People along the western coast of Mexico and along along the Baja need to pay close attention to this dangerous storm. I emphasize that using the current track and intensity forecast, Jimena will be a Category 4 storm when it approaches the tip of Baja and a Category 3 storm when it makes landfall up the coast.

It is worth noting that if Jimena follows the more easterly tracks presented in the computer track forecasts, the remnants of Jimena could impact NM and western Texas.


I'll update this entry further in response to 94L's development.


Hurricane

Updated: 5:21 PM GMT on August 19, 2011

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Adieu to Danny

By: JeffMasters, 9:50 AM GMT on August 29, 2009

Hi, Dr. Rob Carver filling in for Dr. Masters. With the 5AM EST update, NHC has discontinued issuing the tropical storm watches and will no longer issue any updates about the storm. Danny has gone extratropical and has merged with a frontal low off the Carolina coast. The most recent aircraft reconnaissance flight was unable to find a cyclonic circulation or tropical storm force winds in the remnants of Danny.

That isn't to say that Danny's remnants don't pose an element of risk for the East Coast. High surf from large swells is expected along the East Coast. New England can expect to see a lot of rain as Denny's remnants fly by.

Invest 94L

Invest 94L is still out near South America, but it's convective activity seems to be cycling down right now. The global models (GFS and GEM) do pull 94L (to be precise, a feature that could be 94L) north of the Lesser Antilles. Given how the GFDL has performed with other Invests this season, I can wait a day or so before they run the GFDL for this storm.

East Pacific and elsewhere

For those of you wanting to look at tropical storms, Jimena has just formed in the Eastern Pacific basin. The track uncertainty is fairly large, so I'll be keeping an eye on this storm to see if it will impact Mexico or the American Southwest. In the Western Pacific, Tropical Storm Krovahn looks like it will be grazing the Japanese coastline just east of Tokyo Bay.

I'll give Invest94L a chance to see the Sun and then I'll update this entry.

Update:1545 EDT

G'afternoon everybody, Invest94L has perked up a little since this's morning blog entry. In my judgment, the extent and peak magnitude of convection has increased, and scatterometer data shows that Invest94L has a weak cyclonic circulation. Here's the 9Z ascending pass:

Quickscat Pass over Invest94L
Figure 1Ascending Quikscat pass centered over Invest94L at 9Z Aug 29 2009.

There will be a big update tonight after the 00Z model cycle data comes in.

Hurricane

Updated: 6:30 AM GMT on August 30, 2009

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Danny still weak

By: JeffMasters, 7:32 PM GMT on August 28, 2009

Tropical Storm Danny continues to look unhealthy, with an exposed low-level center and the main heavy thunderstorms well to the east. The center is oval instead of circular, which may portend that this center will dissipate and a new center will form under the heaviest thunderstorm activity. Danny has more of the appearance of a subtropical storm than a tropical storm on satellite imagery, and this structure will slow down any potential intensification. The amount of heavy thunderstorm activity has increased in the past few hours, though no thunderstorms have formed near the center. The latest Hurricane Hunter mission found one small spot of 45 mph surface winds between 1 - 3 pm EDT today, so Danny may barely qualify as a tropical storm. Danny's center may have begun moving to the north over the past hour, giving confidence that the storm's strongest winds and rain will stay offshore of North Carolina tonight and Saturday morning.


Figure 1. Afternoon satellite image at 3:03 pm EDT of Danny, showing the exposed swirl of clouds where Danny's center is, well displaced from the heaviest thunderstorm activity to the east. The center is oval-shaped and not circular, the sign of a weak circulation.

The forecast for Danny
With wind shear at 15 knots this afternoon, and forecast to increase to 20 knots tonight and 30 knots Saturday morning, it is unlikely Danny will be able to strengthen to more than a 50 mph tropical storm. Dry air from the upper-level low that has been keeping Danny disorganized continues to be a problem for the storm, as well. Most of the intensity forecast models continue to insist Danny will strengthen, but they have been doing a very poor job forecasting the intensity of Danny. With Danny's heavy thunderstorms all on the east side of the storm, it is unlikely that North Carolina or New England will feel tropical storm force winds from Danny when it scoots past on Saturday. Large swells from Danny creating high surf along the beaches of New England will be the primary hazard from the storm.

Massachusetts hurricane history
Two tropical storms have affected Massachusetts in the past decade, though neither of these storms brought sustained winds of tropical storm force (39 mph) to the state. Tropical Storm Beryl of 2006 just missed Cape Cod as a weak tropical storm with 45 mph winds. Beryl brought wind gusts to tropical storm force to Nantucket Island, and a 1 foot storm surge. Tropical Storm Hermine hit southeast Massachusetts on August 31, 2004, as a minimum-strength tropical storm with 40 mph winds. No land stations in Massachusetts reported tropical storm force winds during Beryl. The last time Massachusetts measured tropical storm force winds was in 1997 during that year's version of Tropical Storm Danny. Chatham recorded sustained winds of 44 mph, and Nantucket had 43 mph winds. The last time Massachusetts had hurricane force winds was in 1991 during Hurricane Bob, which hit Rhode Island as a Category 2 hurricane. Provincetown, Massachusetts measured sustained winds of 98 mph, gusting to 115 mph, and Buzzard's Bay received a 15 foot storm surge.

Invest 94L
The well-organized tropical wave (94L) mid-way between Africa and the Lesser Antilles Islands continues to be a threat to develop into a tropical depression in the next day or two. Water vapor satellite loops show that 94L has moistened the region surrounding it considerably today, and the storm is not ingesting as much dry air as this morning. However, visible satellite loops show only a small amount of heavy thunderstorm activity near the circulation center, which is broad and elongated from east to west. Shear is low, about 10 knots, and is expected to remain low, 5 - 10 knots, over the next five days. The waters are warm enough to support development, 27°C, and are expected to remain in the 27 - 28°C range over the next five days. It appears that 94L needs another 1 - 3 days to develop a well-formed circulation and become a tropical depression, given the favorable environment. NHC is giving 94L a medium (30 - 50% chance) of developing into a tropical depression by Sunday. Most of the models predict 94L will Be affected by two troughs of low pressure over the next week, which will pull the storm far enough north so that it will miss the Lesser Antilles Islands. It is then probable that 94L will be forced to the west again as the high pressure ridge steering the storm builds back in. The possible long-term threat to the U.S. East Coast is impossible to evaluate at this time.

I'm in New York City this weekend for my cousin's wedding, so will not be blogging again until Monday morning. In my absence, wundergound's severe storms expert, Dr. Rob Carver, will be posting in my blog Saturday and Sunday.

Jeff Masters

Hurricane

Updated: 5:21 PM GMT on August 19, 2011

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Danny weakens further; 94L worth watching

By: JeffMasters, 1:54 PM GMT on August 28, 2009

Tropical Storm Danny continues to weaken, and may merely be a tropical depression. Data from the Hurricane Hunters early this morning showed top winds of just 40 mph at the surface, and satellite intensity estimates of Danny's strength are now rating the storm a tropical depression. Danny looks very disorganized on satellite imagery (Figure 1), with the low-level circulation center exposed to view. There is no heavy thunderstorm activity near Danny's center, and only a small area of heavy thunderstorms lying several hundred miles east of the center.


Figure 1. Morning satellite image of Danny, showing the exposed swirl of clouds where Danny's center is, well displaced from the heaviest thunderstorm activity to the east.

The forecast for Danny
With wind shear at 15 knots this morning, and forecast to increase to 20 knots tonight and 30 knots Saturday morning, is is unlikely Danny will be able to strengthen to more than a 50 mph tropical storm. Dry air from the upper-level low that has been keeping Danny disorganized continues to be a problem for the storm, as well. The GFDL and HWRF models continue to insist Danny will strengthen to a strong tropical storm or weak Category 1 hurricane, but this seems very unrealistic given Danny's current struggles. With Danny's heavy thunderstorms all on the east side of the storm, it is unlikely that New England will feel tropical storm force winds from Danny when it scoots past on Saturday. Large swells from Danny creating high surf along the beaches of New England will be the primary hazard from the storm.

Massachusetts hurricane history
Two tropical storms have affected Massachusetts in the past decade, though neither of these storms brought sustained winds of tropical storm force (39 mph) to the state. Tropical Storm Beryl of 2006 just missed Cape Cod as a weak tropical storm with 45 mph winds. Beryl brought wind gusts to tropical storm force to Nantucket Island, and a 1 foot storm surge. Tropical Storm Hermine hit southeast Massachusetts on August 31, 2004, as a minimum-strength tropical storm with 40 mph winds. No land stations in Massachusetts reported tropical storm force winds during Beryl. The last time Massachusetts measured tropical storm force winds was in 1991 during Hurricane Bob, which hit Rhode Island as a Category 2 hurricane. Provincetown, Massachusetts measured sustained winds of 98 mph, gusting to 115 mph, and Buzzard's Bay received a 15 foot storm surge.


Figure 2. Tropical wave 94L, a few hundred miles west of the Cape Verdes Islands.

Invest 94L
A well-organized tropical wave (94L) lies a few hundred miles west-southwest of the Cape Verdes Islands. This disturbance has shown little change this morning. This morning's QuikSCAT pass shows a complete circulation, elongated in the east-west direction, with top winds of 30 mph. Satellite imagery shows only a small amount of heavy thunderstorm activity near the circulation center, but considerable thunderstorm activity south of the center. Shear is low, about 10 knots, and is expected to remain low to moderate, 5 - 15 knots, over the next five days. The waters are warm enough to support development, 28°C, and are expected to remain in the 27 - 28°C range over the next five days. The dry Saharan Air Layer (SAL) lies about 200 miles to the north and west of 94L, and it appears from water vapor satellite imagery that a modest amount of dry air from the SAL is now being ingested into the storm, slowing development. Dry air will continue to slow development until 94L's heavy thunderstorm activity can moisten the atmosphere enough to shield the storm from the Saharan Air Layer. NHC is giving 94L a medium (30 - 50% chance) of developing into a tropical depression by Sunday. The GFDL and SHIPS models both predict 94L will be a Category 1 hurricane five days from now. However, the HWRF model does not develop 94L. Most of the models predict 94L will take a significant northward jog 3 - 5 days from now, in response to a strong trough of low pressure passing to the north (this is the same trough of low pressure that will be recurving Danny out to sea). It is then probable that 94L will be forced to the west again as the high pressure ridge steering the storm builds back in, and 94L will likely be a few hundred miles northeast of the Northern Lesser Antilles Islands seven days from now.

I'll have an update this afternoon.

Jeff Masters

Hurricane

Updated: 5:21 PM GMT on August 19, 2011

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Danny weakens; not likely to become a hurricane

By: JeffMasters, 7:39 PM GMT on August 27, 2009

Tropical Storm Danny has weakened. Data from the Hurricane Hunters between 2 - 3 pm EDT showed top winds of just 40 - 45 mph at the surface. Satellite intensity estimates of Danny's strength have also declined since this morning. Danny's satellite appearance is pretty unconventional, and the storm doesn't look much like a tropical cyclone. The low level circulation center is exposed to view, with the heaviest thunderstorms lying several hundred miles east of the center. The center has remained nearly stationary the past four hours, and has grown less distinct. It is possible that a new center will form up to 100 miles to the east, in the region with the heaviest thunderstorm activity. Danny has already undergone several center relocations over the past 12 hours.


Figure 1. Afternoon satellite image at 3:17 pm EDT of Danny, showing the exposed swirl of clouds where Danny's center is, well displaced from the heaviest thunderstorm activity to the east.

The forecast for Danny
There's not much new to report from the latest set of 12Z model runs for Danny. The models have pretty much maintained their previous forecast, calling for Danny to intensify to a Category 1 hurricane by Saturday morning, and pass close to both Cape Hatteras, NC, and Cape Cod, MA. Given that the models did not correctly forecast Danny's current struggles, I believe it is unlikely that Danny will attain hurricane strength. Dry air from the upper-level low that has been keeping Danny disorganized continues to be a problem for the storm, despite the presence of moderate wind shear of 10 - 15 knots. Danny only has about 24 hours of relatively favorable wind shear left, and that is probably not enough time to intensify into a hurricane. By Friday night, a trough of low pressure will approach the U.S. East Coast and bring high wind shear of 20 - 35 knots. Danny will be close enough to this trough that the trough may be able to feed energy to Danny as the trough converts Danny to an extratropical storm. As a result, Danny may not weaken only slowly. A landfall in Cape Cod, eastern Maine, or Nova Scotia as a tropical storm with 45 - 55 mph winds is a reasonable forecast.


Figure 3. Tropical wave 94L off the coast of Africa.

Invest 94L off the coast of Africa
A well-organized tropical wave (94L) lies a few hundred miles southwest of the Cape Verdes Islands, near the coast of Africa. This disturbance continues to look more organized on satellite imagery. Shear is low, about 10 knots, and waters are warm enough to support continued development. The dry Saharan Air Layer is relatively limited in extent and intensity, so dry air may have only a small inhibiting effect on the wave. Expect continued development of this wave as it moves westward over the next few days. NHC is giving this system a medium (30 - 50% chance) of developing into a tropical depression by Saturday. This system is moving rather slowly, 10 - 15 mph. The GFDL and HWRF models both predict 94L will be a Category 1 hurricane five days from now. These models also predict that 94L will take a significant northward jog 3 - 5 days from now, in response to a strong trough of low pressure passing to the north (this is the same trough of low pressure that will be recurving Danny out to sea).

I'll have an update Friday morning.

Jeff Masters

Hurricane

Updated: 5:21 PM GMT on August 19, 2011

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Danny disorganized, but generating strong winds

By: JeffMasters, 1:55 PM GMT on August 27, 2009

Tropical Storm Danny continues to look disorganized this morning. The low level circulation center is exposed to view, with the heaviest thunderstorms lying several hundred miles east of the center. The center has undergone several relocations over the past 12 hours, and may do so yet again this morning, in order to position itself nearer to the heaviest thunderstorm activity. Despite its disorganization, Danny continues to generate strong winds, with the Hurricane Hunters and QuikSCAT both reporting winds in the 55 - 60 mph range early this morning. There is plenty of dry air in Danny's vicinity interfering with development, thanks to an upper-level trough of low pressure.


Figure 1. Morning satellite image of Danny showing the exposed swirl of clouds where Danny's center is, well displaced from the heaviest thunderstorm activity to the east.

The intensity forecast for Danny
The upper-level low that has been keeping Danny disorganized has weakened and separated from the storm, leaving Danny in a region with moderate wind shear of 10 - 15 knots and a modest amount of dry air. These environmental conditions will remain roughly constant through Friday night. Slow to moderate strengthening of Danny to a Category 1 hurricane should result, and is called for by all of the reliable intensity models. By Friday night, a trough of low pressure will approach the U.S. East Coast and bring high wind shear of 20 - 35 knots through Saturday. Danny will be close enough to this trough that the trough may be able to feed energy to Danny as the trough converts Danny to an extratropical storm. As a result, Danny may not weaken as fast as one might ordinarily expect, given the high levels of wind shear expected on Saturday. A landfall in Cape Cod, eastern Maine, or Nova Scotia with 55 - 75 mph winds is a good bet.


Figure 2. Performance of the main models used to forecast Hurricane Bill. Forecasts for the time periods 24, 48, 72, 96, and 120 hours are shown, with the track errors for each models' forecasts in nautical miles (nm). The statistics are shown for the regular interpolated version of the models used by the NHC forecasters in real time to make their forecasts. The "Consensus" model is the NHC's TVCN consensus, which is the average of at least two of the other models shown here (but not including the Canadian model). The Canadian model had the best performance of any model for Bill, surpassing even the Official NHC forecast. The next best performing models were the GFDL and GFS. Last year's best performing model, the European Center model, was not available for this analysis. Image credit: Dr. Jim Goerss, Navy Research Lab, Monterey.

The track forecast for Danny
Wunderground provides a computer models plot showing the hurricane track forecasts of most of the major models used by NHC to formulate their official forecast (one notable exception: we can't show the European Center ECMWF model, since this model is not freely available). One of the most frequently asked questions I get is, "which model do you trust?" This morning we have several models like the NOGAPS and Canadian calling for Danny to pass very near Cape Hatteras, North Carolina, then over Cape Cod, Massachusetts. The rest of the models foresee Danny missing Cape Hatteras, and continuing on to an encounter with Nova Scotia instead of Cape Cod. According the 2008 NHC forecast verification report, the best performing model during the 2008 hurricane season was the European Model (ECMWF), by a wide margin. The ECMWF out-performed the official NHC forecast, and it is very rare for an individual model to do this. The next best models were the GFDL and HWRF, while the NOGAPS, UKMET, and GFDN did the worst of the major models. The Canadian model was not analyzed, but historically has been among the worst of the models for forecasting hurricanes.

So far this year, the ECMWF has also done well. Unfortunately, the European Center group does not make the output of their hurricane tracking module publicly available, so I cannot present any statistics of their model's performance. Somewhat surprisingly, the Canadian model has also done very well this year. The model received a major upgrade in its physics of the past year, and has performed extremely well in hurricane track forecasts for both the Eastern Pacific and the Atlantic so far this year. In fact, for Hurricane Bill, the Canadian model gave better track forecasts then the NHC did (Figure 2). Danny is a different storm than Bill, and it is possible that the Canadian model will do less well with a storm that is disorganized, like Danny is. Nevertheless, with the Canadian model consistently keeping a Danny's track close to Cape Hatteras and going over southeastern Massachusetts, residents of these areas need to be prepared for possible hurricane conditions from Danny. Given the recent reformation of Danny's center, and the possibility of yet another reformation later today, all of the track models must be viewed with more than the usual amount of doubt. Since the center reformations have been moving Danny's center to the north and east, it may be that the Canadian model's prediction lies too close to the U.S. coast.

For those of you wondering about specific probabilities of getting tropical storm force or hurricane force winds, consult the NHC Wind Probability Product. The 11 am EDT NHC forecast gave Cape Hatteras a 4% chance of seeing hurricane force winds from Danny, and Nantucket, MA, a 7% chance.

For more information on computer models used by NHC
Basics of hurricane forecast models (Dr. Jeff Masters, wunderground.com, updated 2007)
Description of computer models used for hurricane forecasts (NHC, updated 2009)
Description of computer models used for hurricane forecasts (NOAA/AOML)


Figure 3. Tropical wave 94L off the coast of Africa.

Invest 94L off the coast of Africa
A well-organized tropical wave lies a few hundred miles southwest of the Cape Verdes Islands, near the coast of Africa. This wave was designated 94L by NHC this morning. Shear is low, about 10 knots, and waters are warm enough to support development. The dry Saharan Air Layer is relatively limited in extent and intensity, so dry air may have only a small inhibiting effect on the wave. Expect some slow development of this wave as it moves westward over the next few days. NHC is giving this system a low (less than 30% chance) of developing into a tropical depression by Saturday. This system is moving rather slowly, 10 - 15 mph, and it will be at least a week before it approaches the Lesser Antilles Islands. The GFS model develops 94L into a tropical depression early next week.

Special note on using the Canadian model
While the Canadian model has been doing well with hurricane track forecasts this year, the model still does a poor job forecasting the genesis of new tropical cyclones. The Canadian model has a false alarm rate perhaps three times higher than any other model, so one should not believe the Canadian model's regular predictions of new tropical cyclones springing up. You can access output from the Canadian model at Environment Canada or at Florida State University or Penn State.

I'll have an update later today.

Jeff Masters

Hurricane

Updated: 5:21 PM GMT on August 19, 2011

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Tropical Storm Danny not impressive yet

By: JeffMasters, 7:19 PM GMT on August 26, 2009

Tropical Storm Danny is here, born from an African tropical wave that developed a closed circulation and some respectable heavy thunderstorm activity this morning. Data between 2 - 3 pm EDT from the Hurricane Hunters indicates the Danny has a central pressure of 1009 mb, and top surface winds of about 45 mph. Satellite imagery shows that Danny's circulation has gotten stretched and pulled away from the heaviest thunderstorm activity on the storm's east side. It is possible that Danny's center will reform at a different location, closer to the heaviest thunderstorms, so the model forecasts probably have a higher degree of uncertainty than usual at present. Wind shear has dropped to the low range, 5 - 10 knots. The upper-level low Danny is moving underneath has plenty of dry air in it, and this dry air is intruding into Danny's west side, keeping and heavy thunderstorm activity from developing on that side.


Figure 1. Oceanic heat content for August 25, 2009. Regions where the Sea Surface Temperature is below 80°F, or where the shallow waters of the Continental Shelf lie, do not have enough heat content to be plotted, and are shown in white colors. The oceanic heat content along the track of Tropical Storm Danny is below the threshold of 90 kJ/cm^2 typically seen with cases of rapid intensification. The storm position at 8am EDT today is given by the hurricane symbol with the "0" inside it. Subsequent hurricane symbols show the official NHC forecast points from the 11 am EDT forecast today, for 8am Thursday (24 hours), 8am Friday (48 hours), 8am Saturday (72 hours), and 8am Sunday (72 hours). The cold waters stirred up by Hurricane Bill last week will not impact Tropical Storm Danny. Image credit: RSMAS, University of Miami.

The forecast for Danny
As Danny continues to plow through the upper low, the low will weaken, but will dump dry air into the storm through Thursday afternoon, slowing down development. By Thursday night, when Danny should be several hundred miles off the coast of northern Florida, the upper-level low may be weak enough and far enough away that Danny will find itself in a region with light upper level anticyclonic winds, which would favor more rapid development. Ocean heat content (Figure 1) is high enough to support a hurricane, until Danny gets to about New Jersey's latitude. Most of the intensity models, including the GFDL, HWRF, and SHIPS model, forecast that Danny will become a hurricane by Friday. However, this favorable environment will not last long, since a strong trough of low pressure will be approaching the U.S. East Coast on Friday. This trough will bring high wind shear of 20 - 30 knots by Friday night. This trough should be strong enough to turn Danny to the north, then northeast on Saturday. The models have come into better agreement keeping Danny offshore as it passes North Carolina, though the storm is certainly capable of giving the Outer Banks a direct hit. As Danny passes North Carolina, it should start heading north-northeast, with a landfall likely Saturday afternoon or evening somewhere between Massachusetts and Nova Scotia. At that time, Danny is likely to be a strong tropical storm or weak Category 1 hurricane, with winds in the 55 - 80 mph range. The latest runs of the GFDL and GFS model have Danny tracking directly over Martha's Vineyard, where President Obama is on vacation. It will be interesting to see if the president stays on the island for Danny.

For those of you wondering about specific probabilities of getting tropical storm force or hurricane force winds, consult the NHC Wind Probability Product.


Figure 2. Tropical wave newly emerged from the coast of Africa, with some potential for development.

Elsewhere in the Atlantic
A well-organized tropical wave has exited the coast of Africa, and lies a few hundred miles southeast of the Cape Verdes Islands. None of the models are gung-ho about developing this wave, but shear is low, 10 knots, and waters are warm enough to support development. The dry Saharan Air Layer is relatively limited in extent and intensity, so dry air may have only a small inhibiting effect on the wave. Expect some slow development of this wave as it moves westward over the next few days. NHC is giving this system a low (less than 30% chance) of developing into a tropical depression by Friday afternoon.

I'll have an update Thursday.

Jeff Masters

Hurricane

Updated: 5:22 PM GMT on August 19, 2011

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Tropical Storm Danny likely to be named today

By: JeffMasters, 1:25 PM GMT on August 26, 2009

First-light satellite images of the tropical wave (92L) a few hundred miles northeast of the Bahama Islands show that the system has developed an organized surface circulation, and 92L will likely be named Tropical Storm Danny later today. This morning's QuikSCAT pass confirms the presence of a surface circulation, and the satellite saw top winds of 50 mph in a cluster of thunderstorms well east of the center. Wind shear has dropped to the moderate range, 10 - 15 knots. The upper-level low 92L is moving underneath has plenty of dry air in it, and the upper low is injecting this dry air into 92L's west side, keeping and heavy thunderstorm activity from developing on that side. The Hurricane Hunters are in 92L, and have found surface winds up to 57 mph so far this morning. NHC continues to give 92L a high (greater than 50% chance) of developing into a tropical depression by Thursday afternoon.


Figure 1. Water vapor satellite image from 8:15 am EDT 8/26/09 showing dry air associated with an upper-level low pressure system to the west of 92L's center of circulation. Image credit: NOAA/SSD.

The forecast for 92L
As 92L continues to plow through the upper low, the low will weaken, and wind shear is expected to decline to the low range, 5 - 10 knots, by tonight. However, the upper-level low will continue to dump dry, cold air into 92L through Thursday afternoon, slowing down development. By Thursday night, when 92L should be several hundred miles off the coast of northern Florida, the upper-level low may be weak enough and far enough away that 92L will find itself in a region with light upper level anticyclonic winds, which would favor more rapid development. Most of the intensity models, including the GFDL, HWRF, and SHIPS model, forecast that 92L will become a hurricane by Friday. However, this favorable environment will not last long, since a strong trough of low pressure will be approaching the U.S. East Coast on Friday. This trough will bring high wind shear of 20 - 30 knots by Friday night. This trough should be strong enough to turn 92L to the north. The models have come into better agreement keeping 92L offshore as it passes North Carolina, though the storm is certainly capable of giving the Outer Banks a direct hit. As 92L passes North Carolina, it should start heading north-northeast, with a second landfall likely Saturday night or Sunday morning somewhere between Massachusetts and Nova Scotia. At that time, 92L is likely to be a strong tropical storm or weak Category 1 hurricane, with winds in the 55 - 80 mph range. It currently appears that 92L will not bring tropical storm-force winds to the Bahamas or to Florida.

Elsewhere in the Atlantic
The models have been inconsistently predicting formation of several tropical waves coming off the coast of Africa over the next week, and I am going to stop mentioning these forecasts until we get two models predicting the same thing, several model runs in a row.

I'll have an update later today.

Jeff Masters

Hurricane

Updated: 6:07 PM GMT on August 19, 2011

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Hurricane Hunters find 50 - 60 mph winds in disturbance 92L north of Puerto Rico

By: JeffMasters, 8:54 PM GMT on August 25, 2009

The tropical wave (92L) a few hundred miles north of Puerto Rico is generating a large area of surface winds of 50 - 60 mph, according to the latest information from the Hurricane Hunters. Top winds seen so far at their flight level of 1,000 feet were 69 mph, which would make 92L a strong tropical storm if it had a surface circulation.
However, the aircraft has not found a surface circulation, and the satellite appearance shows virtually no change in the amount, intensity, or organization of the storm's thunderstorm activity. Wind shear has dropped to the moderate range, 15 - 20 knots this afternoon, but the upper low 92L is moving underneath is dumping cold, dry air into the region. Dry air continues to get ingested into 92L's thunderstorms, creating strong downdrafts that are robbing 92L of heat and moisture. These downdrafts are creating surface arc clouds that spread out from where the downdraft hits the ocean surface. NHC continues to give 92L a high (greater than 50% chance) of developing into a tropical depression by Thursday afternoon.

The forecast for 92L
As 92L moves underneath the center of the upper low on Wednesday morning, the upper low is expected to weaken, and wind shear is expected to decline to the low range, 5 - 10 knots. However, the upper-level low will continue to dump dry, cold air into 92L through Thursday afternoon, slowing down development. By Thursday night, when 92L should be several hundred miles off the coast of northern Florida, the upper-level low may be weak enough and far enough away that 92L will find itself in a region with light upper level anticyclonic winds, which would favor more rapid development. However, this favorable environment will not last long, since a strong trough of low pressure will be approaching the U.S. East Coast on Friday. This trough will bring high wind shear of 20 - 30 knots by Friday night. This trough should be strong enough to turn 92L to the north. The models disagree substantially on how close 92L will be to the coast at that time. One camp of models, including the NOGAPS, Canadian, UKMET, and ECMWF models, predict 92L will pass very close to the Outer Banks of North Carolina on Friday night or Saturday morning. The GFS, GFDL, and HWRF models keep 92L several hundred miles out to sea. Both sets of models bring 92L north-northeastwards on Saturday, with a track over Massachusetts or Nova Scotia. The intensity forecast for 92L is problematic, since it's eventual strength depends upon how quickly it manages to become a tropical depression. Given that 92L will find itself in a favorable environment for strengthening for about 36 hours this week, and marginal for the remainder of the week, I give the system these odds:

10% chance of never getting a name.
20% chance of becoming a weak tropical storm (40 - 50 mph winds).
40% chance of becoming a strong tropical storm (55 - 70 mph winds).
30% chance of attaining hurricane strength.

Elsewhere in the Atlantic
The ECMWF and UKMET models predict the development of a tropical wave coming off the coast of Africa late this week. The GFS model no longer shows this.

I'll have an update Wednesday morning.

Jeff Masters

Hurricane

Updated: 6:07 PM GMT on August 19, 2011

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Disturbance 92L getting more organized; storm surge basics explained

By: JeffMasters, 2:06 PM GMT on August 25, 2009

A tropical wave (92L) with an increasing amount of heavy thunderstorm activity is located a few hundred miles northeast of Puerto Rico, and is tracking west-northwest at 20 mph. Recent visible satellite imagery shows some increased organization of the storm, with upper-level outflow on the north side and a hint of a surface circulation trying to form near 22N 62W. However, the disturbance is moving underneath an upper-level cold-cored low pressure system, and this upper-level low is generating 20 - 30 knots of wind shear due the strong upper-level winds from the west. The upper low is also dumping cold, dry air into 92L, and this is retarding development of the storm. Dry air is getting ingested into 92L's thunderstorms and creating strong downdrafts that are robbing 92L of heat and moisture. These downdrafts are creating surface arc clouds that spread out from where the downdraft hits the ocean surface (Figure 1). Nevertheless, 92L appears determined to become a tropical depression over the next day or two, and NHC is giving 92L a high (greater than 50% chance) of developing into a tropical depression by Thursday morning.


Figure 1. Visible satellite image for 9:15 am EDT 8/25/09. Dry air from an upper-level low is getting sucked into 92L's thunderstorms, creating strong downdrafts that appear as arc clouds that spread away from where the downdraft hits the surface.

As 92L moves underneath the center of the upper low, wind shear may decline below 20 knots, and this is expected to occur by tonight. The lower shear may allow more development, and the Hurricane Hunters are on call to fly the storm this afternoon, if needed. The upper-level low is expected to move west-southwest over the next two days, and will continue to interfere with the development of 92L through Thursday. By Friday, when 92L should be several hundred miles off the coast of South Carolina, The upper-level low may be far enough south of 92L that the disturbance will find itself in a region with light upper level anticyclonic winds, which would favor more rapid development. A strong trough of low pressure will be approaching the U.S. East Coast at that time, and all of the models predict that this trough will be strong enough to turn 92L to the north. The models disagree substantially on how close 92L will be to the coast at that time, with several models bringing the storm ashore over North Carolina on Friday night and over New England on Saturday night, and other models keeping the storm out to sea. The intensity of 92L at that time is also problematic, with the HWRF model calling for a weak 40 mph tropical storm, and other models predicting a stronger tropical storm. Keep tuned.

Elsewhere in the Atlantic
The GFS model predicts the development of a tropical wave coming off the coast of Africa late this week.

Storm surge basics
I've developed an extensive set of web pages dealing with storm surge, which will include detailed storm surge inundation maps for the entire U.S. coast for each Saffir-Simpson Category hurricane. I'll be running portions of this material in my blog this week, and plan to release the full set of storm surge pages next week.

If you live near the ocean, the storm surge is the most dangerous part of a hurricane's hazards. The high death tolls of the ten deadliest U.S. hurricane disasters, including the Galveston Hurricane of 1900 (over 8000 killed), the Lake Okeechobee Hurricane of 1928 (2500 killed), and Hurricane Katrina of 2005 (1833 killed), were primarily due to the storm surge. The storm surge is water that is pushed onto shore by a hurricane. It is rarely a "wall of water" as often claimed, but rather a rise of water that can be as rapid as several feet in just a few minutes. The storm surge moves with the forward speed of the hurricane--typically 10 - 15 mph. This wind-driven water moving at 10 - 15 mph has tremendous power. A cubic yard of sea water weighs 1,728 pounds--almost a ton. A one-foot deep storm surge can sweep your car off the road, and it is difficult to stand in a six-inch surge. Compounding the destructive power of the rushing water is the large amount of floating debris that typically accompanies the surge. Trees, pieces of buildings, and other debris float on top of the storm surge and act as battering rams that can cave in any buildings unfortunate enough to stand in the way. If you receive an evacuation order for a hurricane storm surge, it is a very good idea to get out sooner rather than later. The storm surge can begin to rise a day before the storm hits, cutting off escape routes when low-lying highways are flooded. This is particularly true along the Gulf of Mexico shore.


Figure 1. Hurricane Katrina's storm surge pours over the 8-foot high north levee of the MRGO/Intra-Coastal Canal, directly under the Paris Road Bridge in eastern New Orleans. The photo was taken by Dan McClosky, the manager of Entergy's Michoud Power Plant. According to an interview at wwltv.com, "there were waves up on top of that, that were probably 15 to 18 foot on top of what you saw from the hurricane protection levee that was out there," McClosky said. Mike Collins of Austin, Texas has put together a nice in-depth description of this photo, which was judged as being authentic on the hoax debunking web site snopes.com.

The storm surge is created by wind, waves, and low pressure
There are three mechanisms that contribute to the storm surge:

1) The action of the winds piling up water (typically more than 85% of the surge).
2) Waves pushing water inland faster than it can drain off. This is called wave set-up, and is not forecast by the SLOSH model as of 2009. Wave set-up is typically 5 - 10% of the surge.
3)The low pressure of a hurricane sucking water higher into the air near the eye (typically 5 - 10% of the surge).

The storm surge depends greatly upon the size and intensity of a hurricane, the angle that it approaches the shore at, how deep the water is close to shore (the slope of the seabed at the coastline), and how fast the hurricane is moving.


Figure 2. Depiction of a fifteen foot hurricane storm surge occurring at high tide of two feet about mean sea level, creating a seventeen foot storm tide. Note that there are 10-foot waves on top of the 17-foot storm tide, so the external high water mark (HWM) left on the outside of structures by this hurricane could be 27 feet or higher. Image credit: NOAA SLOSH Display Training Manual (PDF File).

Definitions: storm tide, storm surge, high water mark
The storm surge is how high above current sea level the ocean water gets. The number we are most interested in regarding storm surge is how many feet above mean sea level (MSL) inundation will occur. This number is the storm tide, not the storm surge. The storm tide is the height of the storm surge above mean sea level (MSL), corrected for the tide. For example, in a location where high tide is two feet higher than mean sea level, and low tide is two feet lower than mean sea level, a 15-foot storm surge would cause a 17-foot storm tide if the hurricane hit at high tide or a 13-foot storm tide at low tide. Keep in mind that on top of the storm surge will be large waves capable of causing severe flooding and battering damage, and these waves are not included in storm surge forecasts. The waves on top of the storm tide break when they reach shallow water, and create an external high water mark (HWM) on structures. The high water mark can be much higher than the storm surge or storm tide. For example, the maximum storm surge of Hurricane Katrina was 27.8 feet in Pass Christian, MS (measured inside a building where waves couldn't reach). However, the highest high water mark was much higher--34.1 feet on the outside of a building in Biloxi, MS, where a high tide of about 1 foot combined with 11-foot high waves on top of the 22-foot storm surge to create the 34.1 foot high water mark.


Figure 3. High water marks on East Ship Island, Mississippi, after Hurricane Katrina in 2005. Left: Bark stripped off a tree with salt-burned pine trees in the background (note the 25 ft (7.65 m) long survey rod for scale). Right: Massive beach and over wash erosion illustrated by damaged and snapped pine trees along the beach. Image credit: Fritz et al., 2007, "Hurricane Katrina storm surge distribution and field observations on the Mississippi Barrier Islands" (PDF File), Estuarine, Coastal, and Shelf Science (2007), doi:10.1016/j.ecss.2007.03.015.

I'll have an update this afternoon.

Jeff Masters


Hurricane

Updated: 6:07 PM GMT on August 19, 2011

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Bill is gone; Invest 92 pops up

By: JeffMasters, 2:08 PM GMT on August 24, 2009

Hurricane Bill is no more. The hurricane swept past Canada's Nova Scotia province Sunday afternoon, then made landfall early this morning in southeastern Newfoundland as a borderline tropical storm/Category 1 hurricane. Bill's waves claimed two lives over the weekend, a 54-year old swimmer that drowned in Florida, and a 7-year old girl in Maine that got swept into the sea by a big wave. The first death of the 2009 Atlantic hurricane season occurred on August 16, when a swimmer drowned in the rough surf from Tropical Storm Claudette at Pananma City Beach, FL.

Nova Scotia misses a direct hit
The center of Bill scooted parallel to the coast of Nova Scotia Sunday afternoon, and never quite came onshore. Since the storm's forward speed was so rapid--about 35 mph--this resulted in a highly asymmetric wind distribution. Since the top winds of a hurricane include the forward motion of the storm, Bill's top winds of 85 mph observed in the offshore, right front quadrant of the storm meant that the winds on the weak side of the storm, over Nova Scotia, were 85 mph minus 35 mph, or just 50 mph. Winds along most of the coast stayed below 39 mph, the borderline for tropical storm-force winds. The strongest winds measured in Canada were at Sable Island, which lies 150 miles offshore of Nova Scotia. Winds on the island hit 61 mph, gusting to 77 mph, between 4 - 5 pm ADT Sunday afternoon. A few islands along the Nova Scotia coast, such as Beaver Island and Hart Island, reported sustained winds of 39 - 40 mph. The big story for Nova Scotia was the waves from Bill. Buoy 44258 at the mouth of Halifax Harbor recorded significant wave heights of 29.5 feet and maximum wave heights of 49 feet as Bill passed 50 miles offshore. The buoy recorded top sustained winds of 35 mph, gusting to 51 mph. The waves combined with a 1.5 - 3 foot storm surge flooded many coastal roads. Buoy 44150, about 160 miles offshore of of the southwest tip of Nova Scotia, was in the east eyewall of Bill between 10 - 11 am ADT, and reported sustained winds of 62 mph, gusting to 85 mph, with significant wave heights of 44 feet. The buoy recorded a maximum wave height of 87 feet, according to Environment Canada. The highest official rain report on Nova Scotia was 2.6" (65 mm) at Yarmouth. Rainfall cause some localized flooding and road damage. Bill's winds cut power to about 40,000 people at the height of the storm. At Peggys Cove, three men were hit by a giant wave but were not hurt. A gift shop and attached home in the village were swept off of their foundation.

Newfoundland gets hit, but damage is minor
The southeast corner of Newfoundland took a direct hit from Bill. The storm made landfall early this morning as a borderline tropical storm/Category 1 hurricane. Top winds on the island were measured at Cape Race, which recorded sustained winds of 58 mph, gusting to 76 mph, between 1:30 and 2:30 am NDT. A storm surge of 1.2 meters (4 feet) was estimated by Environment Canada for Placentia Bay where Bill made landfall. Damage was minor on Newfoundland, with no major flooding reported. Bill dumped up to three inches of rain on Newfoundland.


Figure 1. The eye of Hurricane Bill on August 19 at 2157 UTC, from a NOAA P-3 Hurricane Hunter aircraft flying at 10,000 feet. Photo credit: Jack Parrish of NOAA's Aircraft Operations Center.

Kelvin-Helmholtz instability waves in the eye of Bill
Flight Director Jack Parrish of NOAA's Aircraft Operations Center snapped a cool photo in the eye of Hurricane Bill on Friday, showing the existence of a Kelvin-Helmholtz instability wave (Figure 1). The photo was taken on August 19 at 2157 UTC, from a NOAA P-3 Hurricane Hunter aircraft flying at 10,000 feet. The photo is taken looking WNW towards the eyewall. The towering clouds of the eyewall extend up to 50,000 - 55,000 feet in the photo, and the ocean surface is not visible, due to stratocumulus clouds covering the bottom of the eye. The center of the photo shows that the top of one of these stratocumulus clouds has a feature that looks like a breaking wave in the ocean. Well, that is an example of a breaking wave in the atmosphere known as a Kelvin-Helmholtz instability wave. The atmosphere behaves as a fluid, and thus has wave-like motions. When there is a sudden change of wind speed along the top of a cloud (wind shear), the flow can become unstable and cause breaking waves to form. One can see Kelvin-Helmholtz in the sky several times per year, and several alert wunderphotographers have uploaded photos of these waves over the years. However, it is uncommon to see these waves in the stratocumulus clouds covering the eye of a hurricane.


Figure 2. Water vapor satellite image for 8:15 am EDT 8/24/09. A tropical wave is approaching the Northern Lesser Antilles Islands, but is running into high wind shear from an upper-level cold low to the west of it. Image credit: NOAA/SSD.

Tropical wave approaching Lesser Antilles becomes Invest 92
A tropical wave with a moderate amount of shower activity is moving west-northwest at 20 - 25 mph and is approaching the northern Lesser Antilles Islands. This wave was designated "Invest 92" (92L) by NHC this morning. The wave is under about 20 - 30 knots of wind shear due the strong upper-level winds from the west. These winds are being created by the counter-clockwise flow of air around an upper-level cold-cored low north of Puerto Rico (Figure 2). This low is expected to move west-southwest and slowly weaken over the next two days, allowing shear to drop to the moderate 10 - 20 knot range beginning Tuesday night, according to the SHIPS model. By Wednesday, the upper low is predicted to be weak enough and far enough away from 92L that it will have a chance to develop. Most of the models show some degree of development of 92L by Thursday, when it is expected to be a few hundred miles off the coast of South Carolina. This wave could turn northward and give a wet weekend to New England, though it is too early to be confident of this. NHC is giving 92L a low (less than 30% chance) of developing into a tropical depression by Wednesday morning. The upper-level low will create plenty of wind shear and dump cold, dry air into 92L over the next two days, so Wednesday is probably the earliest we can expect the system to begin organizing into a tropical depression.

Several models predict the development of a tropical wave coming off the coast of Africa late this week.

I'll have an update Tuesday.

Jeff Masters

Hurricane

Updated: 6:08 PM GMT on August 19, 2011

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Bill brushes Massachusetts; Nova Scotia gets pounded

By: JeffMasters, 2:05 PM GMT on August 23, 2009

The winds and waves are dying down in coastal Massachusetts, which took a glancing blow from Hurricane Bill last night and this morning. Bill's center passed about 200 miles southeast of Massachusetts' Nantucket Island, bringing top sustained winds of 24 mph, gusting to 31 mph, at the airport. A storm surge of 1 foot was observed on Cape Cod and Nantucket at high tide. A storm surge of 0.5 feet was reported at Newport, RI, and Boston, MA. President Obama arrives in neighboring Martha's Vineyard today for vacation, and will not want to go swimming--seas of up to 15 feet will continue to batter the shores of southeast Massachusetts. Significant wave heights at Buoy 44008, about 60 miles southeast of Nantucket Island reached 27 feet early this morning. A rainband from Bill set up over Massachusetts, from Boston southwestward, and several reports of 3 - 4 inches of rain came from stations in the rain band (Figure 1).


Figure 1. Radar estimated precipitation from Hurricane Bill. Kingston, MA, received 3.74 inches of rain from Bill. Western Massachusetts got even heavier rain from an approaching cold front.

Bill's impact on Canada
The Canadian Hurricane Center is predicting that Bill will generate a storm surge of 0.5 - 1.0 meters (1.5 - 3 feet) along the coasts of Nova Scotia and Newfoundland today, as the storm races northeast at 30 mph. The surge, when combined with the 5 - 10 meter (16 - 33 foot) waves expected to pound the coast, will cause considerable coastal damage. This is the main threat of Bill to Canada. Bill's highest hurricane-force winds should stay offshore this afternoon as the hurricane passes the heavily populated capital, Halifax. However, winds of 60 - 70 mph will likely impact eastern Nova Scotia, where Bill is expected to make landfall later today, causing considerable tree damage and power outages. Heavy rains of 3 - 4 inches will also cause localized flooding problems. Radar out of Halifax, Nova Scotia shows heavy rain from Bill impacting most of the province, but Bill's center is located well offshore. Buoy 44150 was in the east eyewall of Bill at 9:30 am EDT, and reported sustained winds of 62 mph, gusting to 85 mph, with waves of 40 feet. Bill is expected to make landfall over Newfoundland near midnight tonight, but will have likely weakened to a tropical storm by then.

Links to follow:
wundermap for Nova Scotia
Halifax radar
Canadian Hurricane Center advisories

Elsewhere in the tropics
There are no threat areas in the Atlantic worth mentioning today. Most of the models are calling for the possible development of a tropical cyclone in the waters between the Bahamas and North Carolina on Wednesday or Thursday. There could be two triggers for tropical cyclone formation--the remains of the cold front that pushed off the U.S. East Coast this morning, plus a tropical wave that is currently approaching the northern Lesser Antilles Islands. We will also need to watch the region off the coast of Africa this week.

I'll have an update Monday, when I'll show a remarkable photo taken in the eye of Hurricane Bill by the Hurricane Hunters.

Jeff Masters

Hurricane

Updated: 6:08 PM GMT on August 19, 2011

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Bill batters Bermuda; Canada next

By: JeffMasters, 2:09 PM GMT on August 22, 2009

The winds are dying down on Bermuda, which took a glancing blow from Hurricane Bill last night and this morning. Bill's center passed about 170 miles west of the island, bringing top sustained winds at the Bermuda airport of 46 mph, gusting to 60 mph, at 8:55 pm AST last night. One Bermuda weather station at an elevation of 262 ft recorded a wind gust of 95 mph during a severe thunderstorm last night, resulting in some wind damage that will be surveyed today. Hurricane Bill was undergoing an eyewall replacement cycle last night during its passage west of Bermuda, and the collapse of the inner eyewall meant that the hurricane could not strengthen. An outer rain band has now formed into a new, much larger diameter eyewall. As a result, Bill now has a huge, 50-mile diameter eye. Now that the eyewall replacement cycle is complete, Bill has about a 12-hour window of time to intensify, since the hurricane is crossing the axis of the warm Gulf Stream Current, and wind shear is low, 5 - 10 knots.

Around midnight tonight, Bill will lose its warm waters, and SSTs will decline quickly to 19°C (66°F) Sunday morning. By Sunday afternoon, wind shear will rise to 40 knots as Bill encounters the upper-level westerly winds of the large trough of low pressure that is steering the hurricane to the north. These strong upper-level winds will act to turn Bill to the northeast, and shear the hurricane apart. By the time Bill reaches Nova Scotia, Bill should be approaching tropical storm strength, though it will still be generating huge waves.


Figure 1. Visible satellite image of Bill at from Saturday morning 8/22/09.

Bill's waves
Hurricane Bill continues to generate huge waves, thanks to its large size and the long time it spent at major hurricane intensity. This morning, Bill passed 95 miles east of Buoy 41048, which recorded significant wave heights of 27 feet, and sustained winds of 50 mph. Huge waves battered Bermuda yesterday and today, as seen in the wunderphotos taken by denmar, at the bottom of the blog. Output from NOAA's Wavewatch III model suggests that significant wave heights near Bill's center will peak at 40 feet today. Large swells from Bill have reached most of the U.S. East Coast, and wave heights will increase today. Waves at the Nantucket, Massachusetts buoy were up to 9.5 feet this morning, and the Cape Cod Buoy had 8.5' waves. The big waves affecting the U.S. coast will cause very dangerous swimming conditions, and will likely cause several million dollars in beach erosion damage. Though Bill will bring sustained winds near 40 mph and occasional heavy rain showers to southeastern Massachusetts, it is Bill's waves that are the primary threat of the storm to the U.S.

Elsewhere in the tropics
The tropical wave in the middle Atlantic that NHC mentioned in their Tropical Weather Outlook this morning is falling apart, and there are no threat areas in the Atlantic worth mentioning today. Though the models are not calling for any clear-cut development of any tropical cyclones over the next week, we should keep a close eye on the waters between the Bahamas and North Carolina by Wednesday. There could be two triggers for tropical cyclone formation then--the remains of the cold front that will be pushing off the U.S. East Coast this weekend, plus a tropical wave. The Western Caribbean also may be prime for some development late next week, as well as the region off the coast of Africa.

I'll have an update Sunday.

Jeff Masters

Hurricane

Updated: 6:08 PM GMT on August 19, 2011

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A weakening Bill generating a major wave event for Bermuda and the U.S. coast

By: JeffMasters, 2:29 PM GMT on August 21, 2009

Wind shear and dry air are weakening Hurricane Bill, which may no longer be a major hurricane. However, the hurricane is still large enough and strong enough that it will generate a major wave event for much of the Western Atlantic. Visible and infrared satellite imagery show that the hurricane is no longer as symmetric as it once was, with a squashed oval shape to its cloud pattern. Upper-level cirrus clouds are restricted on the storm's southwest side, indicating that upper-level winds from the southwest are shearing the storm. The University of Wisconsin CIMSS wind shear analysis shows about 10 - 15 knots of southwesterly wind shear impacting Bill. Satellite intensity estimates of Bill's strength show essentially no change since 2am EDT this morning, and Bill is at the borderline of becoming a Category 2 hurricane. There are currently no Hurricane Hunter aircraft in Bill, and the next mission is scheduled for 2pm EDT this afternoon.

Wind shear is forecast to remain low to moderate, 5 - 15 knots, for the next two days, and it is possible Bill may see a relaxation of the wind shear affecting it, allowing some re-intensification today. Sea Surface Temperatures (SSTs) will be plenty warm today, as Bill traverses a region of ocean with SSTs near 29°C. Total ocean heat content is steadily declining, though, as the warm waters Bill is currently traversing do not extend to great depth. By Saturday, SSTs decline to 27.5°C, and intensification becomes less likely.


Figure 1. Visible satellite image of Bill at from Friday morning 8/21/09. Bill had an oval shape, and was missing upper-level cirrus clouds on the southwest side, indicating that wind shear from strong upper-level southwesterly winds was affecting it.

The computer model track forecasts for Bill have been very consistent the past two days, giving confidence that the trough of low pressure developing along the U.S. East Coast this weekend will turn Bill to the north then northeast as expected. Both Nova Scotia and Newfoundland, Canada can expect tropical storm conditions from Bill, which should be rapidly weakening on Sunday afternoon when it makes its closest pass to Nova Scotia. Category 1 hurricane conditions are possible on Nova Scotia if Bill makes a direct hit there. Cape Cod and eastern Maine may receive sustained winds of 35 mph from Bill, just below tropical storm force, but the chance of a direct hit by Bill are slim.

Bill's waves
Hurricane Bill is generating huge waves, thanks to its enormous size and major hurricane intensity. Output from NOAA's Wavewatch III model suggests that significant wave heights near Bill's center will peak at 50 feet by Saturday. Large swells from Bill have reached Bermuda, and are generating waves of 10 - 20 feet in the offshore waters. The Bermuda Weather Service predicts seas will increase to 20 - 30 feet on Saturday as Bill makes its closest approach to the island.

In the U.S., Bill's swells will reach New York's Long Island on this afternoon, and seas will build to 7 - 10' on Saturday and 12 - 16' on Sunday in the near shore waters. By tonight, Bill's swells will be affecting the entire U.S. East Coast from Florida to Maine. Maximum sea heights in near shore waters over the weekend will be about 6 - 8' from Florida to South Carolina, 11 - 14' along the North Carolina coast, 8 - 11' along the mid-Atlantic coast, and 11 - 14' along the coast of Maine. The highest waves along the U.S. coast will occur at Cape Cod, Massachusetts, where waves of 18 - 23' are being forecast by NOAA for Sunday. Bill's high waves will likely cause millions of dollars in coastal erosion damage and create very dangerous rip currents and swimming conditions along the coast.

CloudSat slices through Bill
The CloudSat satellite, launched in 2006, carries the first satellite-based millimeter wavelength cloud radar. It is the world's most sensitive cloud-profiling radar, more than 1000 times more sensitive than current weather radars. It collects data about the vertical structure of clouds, including the quantities of liquid water and ice, and how clouds affect the amount of sunlight and terrestrial radiation that passes through the atmosphere. The satellite has a narrow field of view, so can image only a small portion of the planet each day. About once per year, CloudSat happens to slice through the eye of an Atlantic hurricane. This happened Wednesday, when CloudSat caught a remarkable view of Hurricane Bill (Figure 2).


Figure 2. Top: conventional 89 GHz microwave "radar in space" image of Hurricane Bill on Wednesday, 8/19/09, from the AMSR-E instrument on the Aqua spacecraft. Bottom: cross section through Bill's eye taken at the same time from the CloudSat cloud radar instrument. The CloudSat pass occurs along the red line in the top image. The CloudSat pass runs from south (left side of CloudSat image) to north (right side of CloudSat image). At the time of the image, Hurricane Bill was strengthening into a Category 4 hurricane (135 knot winds, 947 mb pressure), while completing an eyewall replacement cycle. In the 89 GHz Microwave image, half of the eyewall is already completed (half red circle on right side of the image). In the CloudSat image, the southern eyewall shows weak echoes at the low levels, and slopes outward with height. The northern eyewall is much more intense, and a core of high reflectivity echoes extends high into the troposphere, to 16 km altitude, forming a "hot tower". These "hot towers" a characteristic of intensifying hurricanes. Interestingly, the hot tower is tilted into the eye, so that the eyewall does not slope outward with height along the northern eyewall. This is not the typical behavior of a hurricane, and may be due to the unusual strength of the hot tower. Cirrus clouds with a base at 8.5 km can be seen to the south of the hurricane in the CloudSat image. The thin grey line at 5 km marks the 0°C temperature line. Ice particles falling inside the hurricane melt at an altitude just below the 0°C line, creating a "bright band" of orange echoes throughout most of the hurricane. Image credit: NASA/Colorado State University.

Elsewhere in the tropics
There are no threat areas in the Atlantic worth mentioning today. The GFS model calls for a tropical cyclone to develop just off the coast of Africa about seven days from now.

I'll have an update Saturday.

Jeff Masters

Hurricane

Updated: 6:09 PM GMT on August 19, 2011

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Bill weakens, but still generating huge waves

By: JeffMasters, 2:29 PM GMT on August 20, 2009

Hurricane Bill has peaked in intensity, and now shows signs of weakening. Visible and infrared satellite imagery show that the hurricane is no longer as symmetric as it once was, with an oval instead of circular shape to its cloud pattern. Upper-level cirrus clouds are restricted on the storm's southwest side, indicating that upper-level winds from the southwest are shearing the storm. The University of Wisconsin CIMSS wind shear analysis shows about 10 - 15 knots of wind shear impacting Bill. The latest 8:18am EDT eye report from the Hurricane Hunters indicated that the eyewall had a gap in its southwest side, and the pressure had risen 2 mb since last night, to 951 mb. Maximum winds at the surface observed by the SFMR instrument were only Category 2 strength, though winds measured at the aircraft flight level of 10,000 feet still suggested Bill may be a Category 3 hurricane.

Wind shear is forecast to remain low to moderate, 5 - 15 knots, for the next three days, and it is possible Bill may see a relaxation of the wind shear affecting it, allowing re-intensification to Category 4 status. Sea Surface Temperatures (SSTs) will be plenty warm over the next three days, as Bill traverses a region of ocean with SSTs of 28 - 29°C. Total ocean heat content is at a maximum today, and will gradually decline over the next three days.


Figure 1. Visible satellite image of Bill at 1:33 pm EDT Thursday 8/20/09. Bill had an oval shape oriented SW - NE, and was missing upper-level cirrus clouds on the southwest side, indicating that wind shear from strong upper-level southwesterly winds was affecting it.

Water vapor satellite loops continue to show two small "short-wave" troughs of low pressure to the northwest of Bill, and these troughs are continuing to steer Bill to the northwest. The short wave troughs (so called because they have a relatively small amplitude and wavelength) are not strong enough to turn Bill due north, so Bill is expected to miss Bermuda. The official NHC forecast has the radius of tropical storm force winds from Bill barely reaching Bermuda on Saturday, so the island can expect sustained winds in the 35 - 45 mph range for a few hours on Saturday if the hurricane follows the NHC forecast track.


Figure 2. Visible satellite image of Bill's eye zoomed in, taken from NASA's Aqua spacecraft at 10:15am EDT Wednesday August 19, 2009. Image credit: NASA GSFC.

An unusually strong "long wave" trough of low pressure (called long wave because of its large amplitude and wavelength) is expected to develop along the U.S. East Coast this weekend. This trough will turn Bill to the north, and also bring high levels of wind shear in the 40 - 65 knot range on Sunday. The models have moved the forecast landfall point of Bill several hundred miles back and forth to the east and west over the past few days, but mostly agree that Cape Cod and Maine will probably miss a direct hit by Bill. However, these regions are still at the edge of Bill's cone of uncertainty, and a direct strike by Bill at Category 1 or 2 strength is a possibility. However, it is more likely that Bill will come ashore over the Canadian provinces of Nova Scotia or Newfoundland. Bill will be weakening rapidly as it makes landfall, and is likely to be a Category 1 hurricane if it hits Nova Scotia, or strong tropical storm if it hits Newfoundland. If Bill follows the official NHC forecast path, winds on Cape Cod and in eastern Maine are likely to remain below tropical storm force (below 39 mph).

Bill's waves
Hurricane Bill is generating huge waves, thanks to its enormous size and major hurricane intensity. Bill passed about 75 miles southwest of Buoy 41044 this morning, and the buoy recorded sustained winds of 67 mph, gusting to 92 mph, with a significant wave height (the height of the average 1/3 highest waves) of 38.8 feet. Output from NOAA's Wavewatch III model suggests that significant wave heights near Bill's center will peak at 50 feet by Saturday. Large swells from Bill will reach Bermuda this afternoon, increasing seas to 5 - 9 feet, according to the Bermuda Weather Service. Seas will increase to 10 - 20 feet on Friday and 20 - 30 feet on Saturday as Bill makes its closest approach to the island.

In the U.S., Bill's swells will reach New York's Long Island on Friday afternoon, and seas will build to 7 - 10' on Saturday and 12 - 16' on Sunday in the near shore waters. By Friday night, Bill's swells will be affecting the entire U.S. East Coast from Florida to Cape Cod. Maximum sea heights in near shore waters over the weekend will be about 7' from Florida to South Carolina, 11 - 14' along the North Carolina coast, 8 - 11' along the mid-Atlantic coast, and 10 - 11' along the coast of Maine. The highest waves along the U.S. coast will occur at Cape Cod, Massachusetts, where waves of 18 - 23' are being forecast by NOAA for Sunday. Bill's high waves are going to cause millions of dollars in erosion damage and create very dangerous rip currents and swimming conditions along the coast.

Hurricane History of Canada
Canada is no stranger to hurricanes, and receives a hit by a Category 1 or stronger hurricane several times per decade, on average. The most recent hurricane strike on Canada occurred in 2008, when Hurricane Kyle struck the southwestern tip of Nova Scotia, just north of Yarmouth. Kyle was rated a Category 1 hurricane with 75 mph winds at landfall, but damage was limited to uprooted trees, scattered power outages, and minor street flooding in Shelburne. The other hurricane to hit Nova Scotia this decade was much more serious. In 2003, Hurricane Juan made landfall at Halifax, Nova Scotia, as a Category 2 hurricane with 100 mph winds. A record storm surge of 4.9 feet inundated the city's waterfront, resulting in extensive flooding of the Halifax and Dartmouth waterfront properties. A buoy just outside Halifax Harbor measured a significant wave height of 9 meters (30 feet), and maximum wave heights of 20 meters (65 feet). Four people died in the storm. Juan downed a phenomenal number of trees--agriculture specialists estimate that 50 - 100 million trees blew down in Nova Scotia in two hours, with one million downed in Halifax alone. The Canadian Hurricane Center has a nice historical hurricane page with more information and photos.


Figure 3. Close up view of the damage at the Bedford Yacht Club after Hurricane Juan in 2003. Photo: Gary Dunbrack. Image credit: Environment Canada website on Hurricane Juan.

Elsewhere in the tropics
There are no threat areas in the Atlantic worth mentioning today, and no reliable models are calling for tropical cyclone development over the next seven days.

I'll have an update Friday morning.

Jeff Masters

Hurricane

Updated: 6:09 PM GMT on August 19, 2011

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Bill intensifies to Category 4; globe has 5th warmest July on record

By: JeffMasters, 3:28 PM GMT on August 19, 2009

Category 4 Hurricane Bill is now the the fourth strongest tropical cyclone to appear on the planet so far this year, and may grow even stronger. Visible and infrared satellite imagery continue to show an impressive, well-organized, hurricane, with plenty of low-level spiral banding and upper-level outflow well-established on all sides except the west. On Bill's west side, upper-level winds from the west are creating a modest 10 knots of wind shear, which is giving the hurricane a bit of a squashed appearance there.

Wind shear is forecast to remain low to moderate, 5-15 knots, for the next four days. Sea Surface Temperatures (SSTs) will rise steadily from 28.5°C today to 29°C on Friday. Total ocean heat content is at a maximum today, and will gradually decline over the next four days. Bill should be able to take advantage of these favorable conditions a remain a major hurricane the next three days.


Figure 1. Visible satellite image from NASA's Aqua spacecraft at 12:40pm EDT Tuesday August 18, 2009. Image credit: NASA GSFC.

Water vapor satellite loops show a small "short-wave" trough of low pressure to the north-northwest of Bill, and this trough has turned Bill on a more northwesterly track over the past two days. Bill will miss the Lesser Antilles Islands, and the main impact of the hurricane on these islands will be high waves. The short wave trough (so called because it has a relatively small amplitude and wavelength) is not strong enough to turn Bill due north, and Bill is also expected to miss Bermuda. High waves and sustained winds of 20 - 30 mph are the worst that Bermuda is likely to get from Bill.


Figure 2. Visible satellite image of Bill's eye zoomed in, taken from NASA's Aqua spacecraft at 12:40pm EDT Tuesday August 18, 2009. Image credit: NASA GSFC.

An unusually strong "long wave" trough of low pressure (called long wave because of its large amplitude and wavelength) is expected to develop along the U.S. East Coast late this week. This trough will turn Bill to the north, and also bring high levels of wind shear in the 40 - 65 knot range on Sunday. Exactly where this turn occurs is still not clear. The models continue to be in two camps: an eastern camp (GFS, GFDL, HWRF, and ECMWF) that takes Bill 300 - 500 miles east of Cape Cod, and a more western camp (NOGAPS, UKMET) that bring Bill within 150 - 200 miles of Cape Cod. Both sets of models bring Bill ashore over the Canadian provinces of Newfoundland or Nova Scotia. Bill will be weakening rapidly as it makes landfall, and is likely to be a Category 1 hurricane if it hits Nova Scotia, or strong tropical storm if it hits Newfoundland.

Bill's big waves
Large swells from Bill will begin impacting the U.S. East coast from Florida to Maine beginning Friday night or Saturday morning. Seas will build to 5 - 10 feet in the offshore waters from central Florida northwards to South Carolina, and to 10 - 15 feet from North Carolina to Cape Cod. Near shore, waves will be about 40% less. This will cause a significant coastal erosion event along some portions of the coast. The latest run of the NOAA Wavewatch III model suggests that significant wave heights near Bill's center will reach 50 feet on Sunday. Since maximum wave height is typically about a factor of 1.9 greater than the significant wave height (which is the average trough-to-crest height of the top 1/3 largest waves), a few huge waves near Bill's center may reach 95 feet high.

Possible impacts to New England
The current set of computer model runs predicts that the center of Bill will pass Cape Cod, Massachusetts Sunday afternoon or evening. Tropical storm-force sustained winds of 39 mph or greater currently extend out 185 miles to the west of Bill's center, so that if Bill maintains its current wind distribution, Cape Cod could see sustained winds of about 40 mph Sunday night if the models predicting a more westerly path are correct. However, Bill will not keep this same radius of winds. The hurricane will weaken considerably beginning Sunday morning, once the storm gets caught up in the approaching long wave trough. High wind shear of 40 - 65 knots due to strong southwesterly winds aloft will act to compress the hurricane in the east-west direction, keeping the hurricane's strongest winds away from Cape Cod. The highest winds are likely to be no more than 30 mph on Cape Cod from Bill, if the storm follows the track of the western camp of models nearest to the Massachusetts. A few rain squalls may affect coastal Massachusetts, but the main impact of Bill on New England is likely to be coastal erosion from high waves.

Elsewhere in the tropics
The remains of Tropical Storm Ana are bringing scattered heavy rain showers to the Bahamas and Florida today. The remains are disorganized, and are not likely to re-develop. The only model calling for a new tropical cyclone to develop in the Atlantic over the next seven days is the GFS model, which predicts development off the coast of Africa about 7 days from now.

Fifth warmest July on record globally; a cold July in the U.S.
The globe recorded its fifth warmest July since record keeping began in 1880, according to NOAA's National Climatic Data Center. NOAA rated the period January - July 2009 as the sixth warmest such period on record. NASA's Goddard Institute for Space Studies rated July 2009 as the 2nd warmest July on record, behind July of 1998. For the second month in a row, global ocean Sea Surface Temperatures (SSTs) in July were the warmest on record, 0.59°C (1.06°F) above the 20th century average. This broke the previous July record set in 1998. The record July SSTs were due in part to an ongoing El Niño event in the Eastern Pacific, which has substantially warmed a large stretch of the tropical Eastern Pacific Ocean. As El Niño conditions mature during the coming months, near-record global ocean and land temperatures will probably continue. Now that El Niño conditions have been well-established for three months, the atmosphere has begun to heat up in response. It typically takes up to seven months for the atmosphere to heat up in response to ocean heating from an El Niño. This may explain why June of 2009, which independent assessments by NOAA, NASA, and the UK Hadley center agreed was the 2nd or 3rd warmest June on record at the surface, recorded only average satellite-measured temperatures in the lower atmosphere. In contrast, the July satellite-measured temperatures in the lower atmosphere were the 2nd or 3rd warmest on record, in agreement with the assessments that surface temperatures were the 2nd to 5th warmest on record.


Figure 3. Departure of temperature from average for July 2009. Image credit: NOAA/NCDC.

A cold July for the U.S.
For the contiguous U.S., the average July temperature of 23.1°C (73.5°F) was the coolest since 1994, and July temperatures were the 27th coolest in the 115-year record, according to the National Climatic Data Center. Ohio, Illinois, Indiana, Pennsylvania, Iowa, and West Virginia experienced their coolest ever July. Kentucky, Missouri, Michigan, and Wisconsin recorded their second coolest July in history. A strong trough of low pressure parked itself over the eastern portion of the U.S. in July, funneling down plenty of cold air from Canada. In the western U.S., a ridge of high pressure dominated, bringing unusually hot conditions. Arizona recorded its 3rd warmest July on record, and Seattle, Washington recorded its hottest day in history on July 28, notching a 103°F reading. This was 3°F above the previous record set in 1994.

U.S. precipitation was near average in July, with the month ranking 40th wettest in the 115-year record. U.S. tornado activity was above average in July, according to NOAA's Storm Prediction Center. However, no tornado deaths occurred in July.

At the end of July, 14% of the contiguous United States was in moderate-to-exceptional drought. This is a drop from the 19% figure observed at the beginning of the year. These extreme drought regions were exclusively in South and Central Texas.

I'll have an update Thursday morning.

Jeff Masters

Climate Summaries Hurricane

Updated: 6:09 PM GMT on August 19, 2011

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Impressive Bill churning huge waves; New England air pollution episode underway

By: JeffMasters, 2:38 PM GMT on August 18, 2009

Hurricane Bill has popped out an impressive eye, and continues to gather strength over the middle Atlantic. Visible and infrared satellite imagery show a well-organized, symmetric hurricane, with plenty of low-level spiral banding and upper-level outflow channels to the north and south. The spectacular appearance of the storm is evidence of the light wind shear environment that Bill finds itself in.

Wind shear is low, 5 - 10 knots, and is forecast to remain low to moderate, 5-15 knots, for the next five days. Sea Surface Temperatures (SSTs) will rise steadily from 27.5°C today to 29°C on Friday. Total ocean heat content also rises today into Wednesday, and it is expected that Bill will take advantage of these favorable conditions to intensify into a major hurricane. The Hurricane Hunters make their first penetration into Bill this afternoon. The NOAA Hurricane Hunters will be continuously flying Bill for the next three days. They are flying research missions that will feed real-time radar data into an experimental version of the HWRF model to see if this data can improve the model forecasts.


Figure 1. Wave forecast for Hurricane Bill from NOAA's Wavewatch III model. Beginning Saturday (right panel) large waves from Bill are expected to affect most of the U.S. East Coast. By Sunday, the model predicts waves of 10 - 15 feet may impact the offshore waters of New England.

Water vapor satellite loops show that a trough of low pressure is diving down towards Bill, and this trough will be able to turn Bill more to the northwest over the next two days, and Bill will miss the Lesser Antilles Islands. The main impact of Bill on these islands will be high waves. Yesterday, Bill passed just north of Buoy 41041, which recorded significant wave heights of 28.8 feet. Maximum wave height is typically a factor of 1.9 greater than the significant wave height, so Bill was likely generating waves up to 55 feet high. High waves from Bill are propagating across the Atlantic towards the U.S. East Coast, and will arrive there on Saturday, according to NOAA's Wavewatch III model (Figure 1). The highest waves spawned by Bill will affect the New England coast, where waves of 10 - 15 feet in offshore waters can be expected. The waves will cause significant erosion of beaches, and possible damage to shoreline structures.

A much larger trough of low pressure is expected to develop along the U.S. East Coast late this week, turning Bill to the north. Exactly where this turn occurs is still not clear, and both Bermuda and Cape Cod, Massachusetts will be in Bill's 5-day forecast cone of uncertainty. At present, it appears that the Canadian provinces of Newfoundland or Nova Scotia are at greatest risk from a strike by Bill, but New England and Bermuda cannot relax just yet.

Elsewhere in the tropics
The remains of Tropical Storm Ana are bringing heavy rain to Haiti, Cuba, and the Bahamas today, and this activity will spread over South Florida tonight. The remains are disorganized, and are not likely to re-develop. No models are calling for any new tropical cyclones to develop in the Atlantic over the next seven days.


Figure 2. Image from NASA's Terra satellite of air pollution haze over the Northeast U.S. on Monday, August 17, 2009.

First major air pollution episode of the summer for the Northeast U.S.
New England is currently experiencing a far more deadly weather event than a direct hit by Hurricane Bill would likely bring--a large dome of high pressure. The reason? The high pressure system camped over the Northeast U.S. has brought hot temperatures, stagnant air, and the summer's first major air pollution episode.

The event started on Sunday, when a high pressure system with light winds moved over the eastern U.S., limited mixing and leading to stagnation and a buildup of pollutants. Mostly sunny skies and high temperatures also enhanced formation of ground-level ozone gas, a dangerous pollutant. Furthermore, southerly winds brought high humidity into the Northeast, which is conducive to particle pollution formation in the atmosphere. Particle pollution is the most deadly form of air pollution in the U.S. The poor air quality led to issuance of air quality advisories and action days on Monday in more than 30 cities, including New York City, NY; Newark, NJ; Providence, RI; and Portland, ME.

Today's air pollution forecast
Today, similar conditions are expected across much of the region, and Air Quality Index (AQI) levels are forecasted to remain in the Unhealthy for Sensitive Groups (Code Orange) range for many areas in the Northeast. For a complete list of action/advisory days and their locations, visit the EPA AIRNow website.

Health Tip: Cut back on strenuous outdoor exercise when air quality is expected to be poor.

How You Can Help: Choose a cleaner commute - share a ride to work or use public transportation. Bicycle or walk when possible. Combine errands and reduce trips.

Mortality from air pollution
As I discussed in a previous blog post, air pollution is a far more deadly weather hazard in the U.S. than hurricanes. Sure, hurricanes have killed an average of 150 people per year in the U.S., and the "premature deaths" caused by air pollution are only partly attributable to breathing bad air, while drowning in a hurricane's storm surge is entirely due to the hurricane. Nevertheless, a great many children die of pollution-induced asthma attacks who would not have died otherwise, and the mortality due to air pollution in the general population is in the thousands or ten of thousands each year. Outdoor air pollution in the U.S. due to particulate pollution alone was estimated by the EPA in 1997 to cause at least 20,000 premature deaths each year. A 2005 study by EPA scientists (Particulate Matter Health Risk Assessment for Selected Urban Areas) estimated that over 4,700 premature deaths occur each year in just nine cities (Detroit, Los Angeles, Philadelphia, Pittsburgh, St. Louis, Boston, Phoenix, Seattle, and San Jose)--even if those cities all met the current federal standards for particulate matter pollution. Extrapolating these data to the entire nation puts the annual death toll in the tens of thousands--but the EPA has not calculated that total. Some studies have placed the annual pollution death toll in the U.S. at 50,000 to 100,000 (Dockery, D.W., and C.A Pope III. Acute Respiratory Effects of Particulate Air Pollution. Annual Review Public Health, 1994, vol. 15,107-32.) The death toll is much higher in other parts of the world, where air pollution standards are not as stringent. Globally, about 800,000 people per year die prematurely due to outdoor air pollution, according to a 2005 study published in the Journal of Toxicology and Environmental Health. This represents about 1.2 percent of total annual global deaths.

In the debate over the costs of switching over the cleaner energy sources, the huge costs and deaths attributable to air pollution are often ignored. Sure, it will be costly to move away from fossil fuels, but let's not forget that the price per gallon we pay at the pump does not include the billions in medical costs we pay for the effects of air pollution.

I'll have an update Wednesday morning.

Jeff Masters

Air and Water Pollution Hurricane

Updated: 9:34 PM GMT on July 13, 2011

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Claudette hits Florida; Ana approaches Puerto RIco; Bill becomes our first hurricane

By: JeffMasters, 2:10 PM GMT on August 17, 2009

Tropical Storm Claudette made landfall at about 1:15 am EDT near the eastern end of Santa Rosa Island, just southeast of Fort Walton Beach in Florida. Claudette's top winds were around 50 mph. A Personal Weather Station in Eastpoint, FL recorded sustained winds of 49 mph, gusting to 66 mph last night. So far, the rain from Claudette has had a tough time penetrating inland (Figure 2). Heavy rains of 3 - 4 inches have been confined to a narrow strip of coast, and Claudette is unlikely to cause any major flooding. Apalachicola received just over 4 inches of rain so far from Claudette. Radar animations out of the Florida Panhandle show that heavy rains continue along the coast in association with a main spiral band of Claudette, and these rains will gradually subside today.


Figure 1. Radar reflectivity image of Tropical Storm Claudette as it approached landfall just southeast of Fort Walton Beach shortly after midnight on 8/17/09.

The tropics featured a rare triple threat the past two days--simultaneous named storms beginning with the letters A, B, and C. The last time this occurred was in the slow-starting 1984 hurricane season, when Tropical Storms Arthur, Bertha, and Cesar were all active on September 1. This year's A, B, and C storms all got their names in just a 33 hour span. This is not a record, since in 1995, three tropical storms--Humberto, Iris, and Jerry--got their names in a 27-hour span (thanks to NOAA's Ryan Sharp for looking up this stat).


Figure 2. Total precipitation estimated by radar for Claudette, as of 3:28pm EDT 8/17/09.

Ana not dead yet
Tropical Depression Ana continues to cling to life, and is now approaching landfall in Puerto Rico. Radar animations from the San Juan, Puerto Rico radar show a surface circulation just southeast of the island, with some low-level spiral banding trying to develop to the south. Recent satellite images also show a rejuvenation of the heavy thunderstorm activity near Ana's center, as the storm regroups from being nearly torn apart yesterday. Ana has already dumped up to 4 inches of rain along the north coast of Puerto Rico, according to radar-estimates.

It is unlikely that Ana will survive past today, however, since the storm will move over both Puerto Rico and the rugged terrain of Hispaniola. The high mountains of these islands should act to disrupt the relatively small and fragile circulation of Ana. None of the computer models foresee that Ana will survive passage over Hispaniola. The Dominican Republic can expect 3 - 6 inches of rain from Ana, and Haiti can expect 1 - 3 inches.


Figure 3. Total precipitation estimated by radar from Ana for Puerto Rico.

Bill becomes the first Atlantic hurricane of 2009
Hurricane Bill continues to gather strength, and is now the first hurricane of the 2009 Atlantic hurricane season. An eye has appeared on visible and infrared satellite imagery, and Bill is displaying an impressive symmetry, with plenty of low-level spiral banding.

Wind shear is low, 5 - 10 knots, and is forecast to be in the low range through Wednesday. With Sea Surface Temperatures only 27°C today, substantial intensification may not occur until Tuesday and Wednesday, when SSTs warm to 28 - 29°C and ocean heat content sharply increases. By Thursday, Bill is expected to leave the favorable upper-level wind environment it currently finds itself in, and moderate shear of 15 - 20 knots may limit further intensification.

Water vapor satellite imagery shows that there is a modest trough of low pressure in the upper atmosphere near 50°W longitude, that Bill is currently approaching. All of the computer models except the UKMET predict that this trough will be strong enough to turn Bill more to the northwest so that the hurricane misses the Lesser Antilles Islands. The UKMET predicts the trough will not affect Bill much, and that the hurricane will pass through or just north of the islands on Thursday. For now, the UKMET solution is being discounted, since the trough at 50W appears substantial enough on satellite imagery to be able to turn Bill more to the northwest.

A much larger trough of low pressure is expected to develop along the U.S. East Coast late this week, turning Bill even more to the northwest. Most of the models predict Bill will pass very close to Bermuda on Saturday as a result. The HWRF model predicts Bermuda will receive a direct hit at Category 4 strength. Until Bill interacts with the small trough at 50°W, it is too early to be confident of the potential threat to Bermuda. By Tuesday, we should have a much better idea of the threat. Likewise, I would like to see the UKMET model come around in line with the other models before dismissing the possible threat to the U.S. East Coast. It currently appears that Bill will miss the U.S. East Coast, but that a strike on the Canadian provinces of Newfoundland or Nova Scotia is possible.

I'll have an update Tuesday morning, or possibly this afternoon.

Jeff Masters

Hurricane

Updated: 6:18 PM GMT on January 05, 2012

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Ana, Bill, and Claudette--a rare early season triple threat

By: JeffMasters, 8:58 PM GMT on August 16, 2009

After an unusually slow start to the Atlantic hurricane season, the tropics exploded into action this weekend. We have a rare triple threat this afternoon--simultaneous named storms beginning with the letters A, B, and C. The last time this occurred was in the slow-starting 1984 hurricane season, when Tropical Storms Arthur, Bertha, and Cesar were all active on September 1. This afternoon, the Hurricane Hunters confirmed than Tropical Storm Claudette had formed in the Gulf of Mexico. Radar animations out of the Florida Panhandle show a small but well-organized tropical storm with plenty of heavy thunderstorm activity, and improving low-level spiral banding. Claudette has built an eyewall that is 1/3 complete on the east side of the center. Some moderate wind shear from upper-level southwesterly winds is keeping Claudette from forming much heavy thunderstorm activity on its west side. Most of the rain is on the right side of the storm, and regions to the left of where Claudette comes ashore will get only a maximum of 1 - 2 inches of rain. Satellite loops show an area of intense thunderstorms with cold cloud tops continuing to expand near the storm's center.


Figure 1. Current short-range radar out of the Florida Panhandle.

Claudette developed literally overnight, and has enough time over water to be a 60 - 65 mph tropical storm by the time it makes landfall tonight along the Florida Panhandle. Claudette reminds me of 2007's Hurricane Humberto, which became a hurricane just 24 hours after first appearing as a tropical depression. By the time Claudette makes landfall, it will have been in existence just 18 hours since the first advisory. This is not enough time to strengthen into a hurricane. If Claudette had had another twelve hours over the 30°C waters of the Gulf of Mexico, it would likely have been a hurricane. Given that the storm is so small, storm surge flooding should not exceed 3 - 5 feet, and will not be the major hazard from Claudette. Inland flooding from heavy rains of 3 - 6 inches is likely to be the main threat from the storm.

Ana near death
The Hurricane Hunters are in Tropical Storm Ana, and have found that the storm is very disorganized, and may not survive another day. Dry air continues to plague the storm, thanks to the large area of Saharan air the storm is embedded in. Ana's very rapid forward motion of 20 - 25 mph has stretched out the storm's circulation from circular to elliptical. This non-circular flow around the center has made the heavy thunderstorm activity less organized, and it may be difficult for the storm to hold together much longer. The outer rain showers from Ana are now on radar out of Martinique.

Nearly all of the computer models forecast Ana will dissipate over the next two days. If there is anything left of the storm by the time it encounters the rugged terrain of Hispaniola, that should finish Ana off. At this time, it does not appear that Ana will be moist enough to cause a major flooding disaster on Hispaniola.

Tropical Storm Bill continues strengthening
Tropical Storm Bill continues to gather strength in the middle Atlantic Ocean, and appears poised to become a major Cape Verdes-type hurricane later this week. Water vapor satellite loops show that Bill has developed a core region of heavy thunderstorms that should be fairly impervious to the dry Saharan air to its northwest, and the storm should be able to start building an eyewall tonight.

Wind shear is low, 5 - 10 knots, and is forecast to be in the low range for the next three days. With Sea Surface Temperatures at 27.5°C, and ocean heat content sharply increasing 2 - 3 days from now, Bill should be able to intensify to a major Category 3 or 4 hurricane by Wednesday. At that time, some increasing shear may limit further intensification.

The big news is that our most reliable computer model from last year--the ECMWF model--appears to have made the right call yesterday, forecasting that a major trough of low pressure would develop along the U.S. East Coast, turning Bill more to the north. All of the other models have now jumped on the ECMWF bandwagon, forecasting that Bill will pass well north of the Lesser Antilles Islands. The UKMET model, which was resisting forecasting the northward turn, has now joined the other models in its latest 12Z run. The main drama with Bill this week will be to see how close it passes to Bermuda. Several models have Bill passing within 200 miles of Bermuda on Saturday. It is too early to be confident that Bill will miss the U.S. East Coast.

I'll have an update Monday.

Jeff Masters

Hurricane

Updated: 6:18 PM GMT on January 05, 2012

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Ana, Bill,--and Claudette?

By: JeffMasters, 1:52 PM GMT on August 16, 2009

After one of the slowest starts to hurricane season in the past twenty years, the hurricane season of 2009 has exploded with activity over the past 48 hours. This morning, Tropical Depression Four joined Ana and Bill in the Atlantic, and appears poised to become Tropical Storm Claudette later today. Indeed, this may already be Claudette, as morning's QuickSCAT pass showed top winds of 45 mph. NEXRAD radar animations out of Tallahassee, FL, show a small but well-organized tropical cyclone with plenty of heavy thunderstorm activity, and improving low-level spiral banding. Satellite loops show an area of intense thunderstorms with cold cloud tops expanding near the storm's center.


Figure 1. Current long-range radar out of Tallahassee, FL.

It's pretty amazing (and a little unnerving) how quickly this storm sprang up. TD 4 developed literally overnight, and has the potential to be a strong tropical storm by the time it makes landfall tonight along the Florida Peninsula. TD 4 reminds me of 2007's Hurricane Humberto, which became a hurricane just 24 hours after first appearing as a tropical depression. I don't think TD 4 has time to reach hurricane strength, since wind shear as diagnosed by the SHIPS model was moderate, 10 - 15 knots, but it does have time to strengthen into a 50 - 60 mph tropical storm before landfall. Given that the storm is so small, storm surge flooding should not exceed 3 -5 feet, and will not be the major hazard from TD 4; inland flooding from heavy rain of 3 - 6 inches is likely to be the main threat from the storm.

Tropical Storm Ana continues to struggle
Tropical Storm Ana continues to struggle. Dry air continues to plague the storm, thanks to the large area of Saharan air the storm is embedded in. Ana's heavy thunderstorms are limited to just one small spot near the center. Wind shear appears to have lessened some, though, since yesterday. Thunderstorms were not able to form at all near the center yesterday, since strong wind shear tends to blow a storm's heavy thunderstorms to one side of the storm, exposing the low-level center to view. With heavy thunderstorms building near the center this morning, shear appears to be less of a problem for the storm. Top winds seen by this morning's QuikSCAT pass were about 35 mph. The outer rain showers from Ana should appear on radar out of Martinique today.

Shear is low (5 - 10 knots), and is forecast to remain low for the next two days. SSTs are warm, 28°C, and will warm further over the next two days. However, there is so much dry air around Ana that significant strengthening appears unlikely. Nearly all of the models predict Ana will dissipate sometime in the next three days, though the HWRF and GFDL predict that this will happen because Ana will move over the rugged terrain of Hispaniola. Ana is likely not strong enough to survive an encounter with the big island that the Dominican Republic and Haiti share. At this time, it does not appear that Ana will be moist enough to cause a major flooding disaster on Hispaniola.


Figure 2. Water vapor image from this morning showing the large area of dry, Saharan air surrounding Ana, and lying to the northwest of Tropical Storm Bill. Image credit: NOAA/SSD

Tropical Storm Bill gathers strength
Tropical Storm Bill is gathering strength in the middle Atlantic Ocean, and appears poised to become a powerful Cape Verdes-type hurricane later this week. QuikSCAT data from this morning shows a large circulation with top winds of 35 - 40 mph. Water vapor imagery (Figure 2) shows that there is some dry air to the northwest of Bill, and this dry air is being drawn into Bill's center, slowing intensification. It will likely take another day before Bill can moisten the atmosphere enough to ward off the dry air, and allow more significant intensification.

Wind shear is low, 5 - 10 knots, and is forecast to be in the low to moderate range, 5 - 15 knots, for the next five days. Sea Surface Temperatures are about 27.5°C, and will remain in the 27.5 - 28°C range the next four days, then warm substantially as the storm nears the Lesser Antilles Islands. The combination of low wind shear and sufficiently warm SSTs should allow Bill to intensify steadily to hurricane strength by Wednesday.

The big news is that our most reliable computer model from last year--the ECMWF model--appears to have made the right call yesterday, forecasting that a major trough of low pressure would develop along the U.S. East Coast, turning Bill more to the north. All of the other models--with the notable exception of the UKMET model--have now jumped on the ECMWF bandwagon, forecasting that Bill will pass well north of the Lesser Antilles Islands. The UKMET has Bill hitting the islands, but is being discounted since it is an outlier. It currently appears that the trough approaching the U.S. East Coast will be strong enough to recurve Bill before it reaches the U.S., though it is too early to be confident of this. Several of the longer range models show Bill passing near Bermuda or Nova Scotia, Canada.

I'll have an update later today.

Jeff Masters

Hurricane

Updated: 6:18 PM GMT on January 05, 2012

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Ana and TD 3 take aim at the Lesser Antilles

By: JeffMasters, 4:17 PM GMT on August 15, 2009

Tropical Storm Ana was born this morning, when the remnants of Tropical Depression Two made a comeback and organized into the first tropical storm of the 2009 Atlantic hurricane season. Ana is the latest first named storm of the season since Hurricane Andrew got its name on August 17, 1992. The two storms have some similarities, as Andrew formed in the same part of the ocean, and also struggled in its early days with high wind shear and dry air. Let's hope the similarities end there.

Ana is struggling this afternoon. After an modest burst of heavy thunderstorm activity prompted NHC to upgrade Ana to a tropical storm early this morning, Ana has run into strong upper-level winds from the west that are creating high wind shear. This shear was not forecast, and it is not clear how long it will last. The shear has acted to drive dry air into the core of Ana, destroying almost all of Ana's heavy thunderstorms. The low-level center of the storm is now exposed to view, something that often foreshadows the death of a storm. It is possible the shear will destroy Ana, and several models (the GFS and ECMWF) forecast this may be the case. However, the shear forecast calls for shear to drop into the low range, 5 - 10 knots, tonight through Tuesday. If the shear does drop as forecast, Ana should be able to moisten the atmosphere around it sufficiently to protest itself from the dry Saharan air that surrounds it (Figure 1). SSTs are 27°C today, and will increase to 28°C by Sunday. By the time Ana moves into the Bahamas, total ocean heat content rises steeply (Figure 2), and rapid intensification of Ana is possible, if the shear and dry air haven't disrupted the storm. The intensity forecast models, for the most part, predict a steady intensification of Ana to the threshold of hurricane strength five days from now. The HWRF model is on the strong side, predicting a Category 2 hurricane. The GFDL predicts a weak tropical storm five days from now, but that is because the model has Ana passing over the rugged terrain of Hispaniola, something the other models do not predict. In summary, the intensity forecast for Ana has higher than usual uncertainty, and I give equal chances that the storm will be a hurricane--or non-existent--four days from now.


Figure 1. Water vapor image from this morning showing the large area of dry, Saharan air surrounding Ana, and lying to the north of Tropical Depression Three. Image credit: NOAA/SSD

Tropical Depression Three forms, could be Bill later today
QuikSCAT data from this morning and satellite loops revealed that the tropical wave (90L) in the middle Atlantic has finally developed a well developed surface circulation and can be classified as Tropical Depression Three. Recent satellite imagery suggests that TD 3 may already be Tropical Storm Bill. Water vapor imagery (Figure 1) shows that TD 3's center consolidated a few hundred miles south of the dry air of the Saharan Air Layer (SAL). Thus, the storm should not be affected by dry air and dust as much as Ana has been. Ana may also act to moisten the atmosphere in front of TD 3, helping protect the storm from the SAL as it edges farther north over the the three days.



Figure 2. Heat content of the ocean, in kJ per square cm. Oceanic heat content steadily increases for Ana and TD 3 as they approach the Lesser Antilles Islands. Oceanic heat content levels of 90 kJ per square cm are frequently associated with rapid intensification of hurricanes. Image credit: University of Miami.

Wind shear is moderate, 15 knots, but is forecast to fall to 10 - 15 knots on days 2 - 5. Sea Surface Temperatures are about 27.5°C, and will remain in the 27.5 - 28°C range the next five days. The combination of low wind shear and sufficiently warm SSTs should allow TD 3 to intensify steadily, and I expect the storm will be at hurricane strength by Wednesday, when it will be near the northern Lesser Antilles Islands. Most of our reliable intensity models strengthen TD 3 into a hurricane by Wednesday. Oceanic heat content (Figure 2) increases sharply just before the islands, so TD 3 could be intensifying rapidly as it moves through or just north of the Lesser Antilles on Thursday. TD 3 consolidated farther south than expected, so the track models calling for a more northerly path were probably incorrect. In particular, the ECMWF model, which had TD 3 turning sharply northwestward and missing the Lesser Antilles Islands, was probably much too far to the north in this morning's 00Z run. TD 3 will probably pass very close to the northern Lesser Antilles islands on Wednesday and Thursday.

I'll have an update Sunday.

Jeff Masters

Hurricane

Updated: 6:14 PM GMT on August 19, 2011

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Disturbance 90L close to tropical depression strength; TD 2 may rise again

By: JeffMasters, 2:02 PM GMT on August 14, 2009

A strong tropical wave (90L), with a large circulation and a moderate amount of heavy thunderstorm activity, is a few hundred miles southwest of the Cape Verdes Islands, off the coast of Africa. This morning's QuikSCAT pass shows that 90L has a large circulation, but this circulation is not yet well-formed, as it is elongated along an east-west axis. Satellite imagery from the European METEOSAT satellite show that the heavy thunderstorms associated with 90L are gradually becoming better organized, with some spiral banding evident on the south side of the storm. Satellite intensity estimates from NOAA already put 90L at tropical depression strength with 30 mph winds, but the storm does not yet have a well enough formed circulation to be classified as a depression. Water vapor imagery shows that since 90L is forming several hundred miles south of the dry air of the Saharan Air Layer (SAL), the storm should not be affected by dry air and dust as much as Tropical Depression Two was. However, there is some dry air from the SAL being sucked into the circulation of 90L, and this is retarding its development.

Wind shear is moderate, about 10 - 15 knots, but is forecast to fall to the low range, 5 - 10 knots, later today. Shear is then forecast to remain low for the next five days. Sea Surface Temperatures are about 28°C, and will remain in the 27 - 28°C range the next five days. The combination of low wind shear and sufficiently warm SSTs should allow 90L to develop into a tropical depression by Saturday, and potentially reach hurricane strength by Wednesday. Most of our reliable models strengthen 90L into a hurricane by Wednesday, when the storm is expected to be near the northern Lesser Antilles Islands. One important outlier is the ECMWF model, which has 90L passing to the north of the islands, and eventually recurving out to sea east of Bermuda. The ECMWF was by far the most reliable model for forecasting hurricane tracks last year. However, all of our models are pretty unreliable going out 5 days in advance for systems that have yet to reach tropical depression strength. Once 90L becomes a tropical depression and develops a well-formed circulation, the models will have a better handle on forecasting where it will go.


Figure 1. The remnants of Tropical Depression Two (left side of image) and tropical wave 90L (right side of image).

Tropical Depression Two may come back
Tropical Depression Two died yesterday, but may rise again. Satellite loops of the remnants of the storm show that heavy thunderstorms are again attempting a comeback near the axis of what is now a tropical wave. However, dry air continues to interfere with the development of the thunderstorm activity, and moderately high wind shear of 15 - 20 knots is also inhibiting the process. Wind shear over the remnants of TD 2 is expected to remain in the moderate range, 10 - 20 knots, over the next four days. Sea Surface Temperatures (SSTs) are 27°C, but will warm to 28°C two days from now. There is plenty of dry, stable air from the Saharan Air Layer (SAL) to the north and west that will continue to cause the storm problems. The relatively cool SSTs and dry air mean that any re-development of the storm will be slow to occur. NHC has given the system a medium (30 - 50% chance) of becoming a tropical depression again by Sunday morning. Only one reliable model, the NOGAPS, is predicting regeneration of TD 2. The NOAA Hurricane Hunters are flying research missions into the remnants of TD 2 today and Saturday.

I'll have an update Saturday morning, or earlier, if significant developments occur.

Jeff Masters

Hurricane

Updated: 6:14 PM GMT on August 19, 2011

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TD 2 nearly dead; African disturbance 90L gathering strength

By: JeffMasters, 2:38 PM GMT on August 13, 2009

Tropical Depression Two is near death, but is still worth watching. The dry, Saharan air to the north and west of the depression, combined with moderately high levels of wind shear of 15 - 20 knots, have almost completely destroyed all of TD 2's heavy thunderstorms. Satellite loops of the storm show a well-formed circulation, but almost no heavy thunderstorm activity.

Wind shear over TD 2 is expected to remain in the modereate range, 10 - 20 knots, over the next five days. Sea Surface Temperatures (SSTs) are 27°C, but will warm to 28°C three days from now. There is plenty of dry, stable air from the Saharan Air Layer (SAL) to to TD 2's north and west that will continue to cause the storm problems. The relatively cool SSTs and dry air mean that TD 2 will not be able to intensify quickly, and some of the models indicate the TD 2 may get destroyed in the next day or two. However, several models still predict TD 2 will become a tropical storm. The HWRF model predicts TD 2 will become a hurricane five days from now, but this seems unlikely given the dry air and relatively high wind shear affecting the storm.


Figure 1. Tropical Depression Two (left side of image) and tropical wave 90L (right side of image).

African tropical wave 90L
A strong tropical wave with a large circulation and plenty of heavy thunderstorm activity is a few hundred miles south of the Cape Verdes Islands, off the coast of Africa. NHC dubbed this disturbance 90L this morning. This morning's QuikSCAT pass shows that 90L has a very large circulation, and top winds of about 30 mph. Satellite imagery from the European METEOSAT satellite show that the heavy thunderstorms associated with 90L are in two major bands, to the north and to the south of the center. There is no heavy thunderstorm activity near the center yet, and this would have to happen before 90L can be named Tropical Depression Three. Water vapor imagery shows that since 90L is forming several hundred miles south of the dry air of the Saharan Air Layer (SAL), the storm should not be affected by dry air and dust as much as Tropical Depression Two has been. Wind shear is about 20 knots over 90L, and is forecast to remain in the moderate range, 10 - 20 knots, over the next five days. Sea Surface Temperatures are about 28°C, and will remain in the 27 - 28°C range the next five days, which are high enough above the 26°C threshold for tropical cyclone formation to allow some slow development to occur. The GFS and ECMWF models continue to predict the development of this wave, though they are now less aggressive about intensifying it than they were in earlier runs. The consensus among the reliable HWRF, GFDL, GFS, and ECMWF models is to bring 90L to point near or just north of the northernmost Lesser Antilles Islands 6 - 8 days from now. The storm could be at hurricane strength by then, as forecast by the SHIPS intensity model.

I'll have an update Friday morning.

Jeff Masters

Hurricane

Updated: 6:14 PM GMT on August 19, 2011

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TD 2 struggling; new African wave bears watching

By: JeffMasters, 2:19 PM GMT on August 12, 2009

Tropical Depression Two is struggling. The depression still has a good chance of becoming the Atlantic hurricane season's first named storm this week, but the dry, Saharan air that surrounds the storm, combined with moderately high levels of wind shear, are interfering with the storm's organization. Satellite loops of the storm show that heavy thunderstorm activity is limited to the west side of the depression. This is due to strong winds out of the southeast that are creating a moderate 15 - 20 knots of wind shear. The intensity and coverage of these thunderstorms is meager, thanks to the large amount of dry air to TD 2's north that is getting wrapped into the circulation. This morning's QuikSCAT pass showed a well-formed surface circulation, with top winds of about 30 - 35 mph.

Wind shear over TD 2 is expected to remain in the moderate range, 10 - 20 knots, over the next five days. Sea Surface Temperatures (SSTs) are a marginal 26 - 27°C, and will not change much over the next three days. There is plenty of dry, stable air from the Saharan Air Layer (SAL) to to TD 2's north that will continue to cause the storm problems. The relatively cool SSTs and dry air mean that TD 2 will not be able to intensify quickly. However, it does appear likely that TD 2 has enough going for it that it will eventually be able to become Tropical Storm Ana sometime in the next three days. Most of the computer models show some weak development, but none of them predict TD 2 will become a hurricane. It is unusual for storms forming this far north to make it all the way across the Atlantic to hit the Lesser Antilles Islands. For now, TD 2 doesn't concern me as much as the new tropical wave to its southeast, just off the coast of Africa.


Figure 1. Tropical Depression Two (left center of image) and a new tropical wave (right side of image), newly emerged from the coast of Africa.

Elsewhere in the Atlantic
A strong tropical wave with some rotation and plenty of heavy thunderstorm activity left the coast of Africa yesterday, and is located a few hundred miles south of the Cape Verdes Islands. The GFS and ECMWF models continue to predict the development of this wave late this week. Both of these models indicate that the wave would track further south than Tropical Depression Two, and could impact the Lesser Antilles Islands in about seven days. This storm bears close attention over the coming days.

Two other tropical waves, one that just passed through the Lesser Antilles Islands, and one about 300 miles east of the islands, are mentioned in NHC's Graphical Tropical Weather Outlook. Both of these waves have very limited heavy thunderstorm activity that is not increasing, and are not a threat to develop over the next two days. None of the computer models develop either of these waves.

Felicia moves through Hawaiian Islands as a tropical depression
Tropical Storm Felicia weakened to a tropical depression before passing through the Hawaiian Islands yesterday. The storm brought scattered heavy rains of 1 - 2 inches to the islands, and no damage was reported. Felicia is only the 10th tropical cyclone of tropical depression strength or higher to hit the islands since 1950. Four other tropical cyclones have passed within 75 miles of the islands during that time period.

Be careful clicking on external links in blog comments
A comment may have been posted on my blog over the weekend containing links to a malicious web site that infected some people's computers when they clicked on the link. The exact link responsible has eluded us thus far, and we are also investigating the possibility that a bad banner ad was responsible. To provide an extra layer of protection, we've installed software on the blogs that will now warn users that they are leaving the site when they click on an external link, so that one has do an extra click to go to an external site. This will give users an opportunity to think twice before clicking on a potentially dangerous link. I apologize to anybody who may have been affected by the attack. Please submit a support ticket to help us out in the future for any issues of this nature.

I'll have an update Thursday morning.

Jeff Masters

Hurricane

Updated: 6:14 PM GMT on August 19, 2011

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TD 2 forms in the Atlantic; hundreds feared dead from Typhoon Morakot; Felicia hits

By: JeffMasters, 1:55 PM GMT on August 11, 2009

Tropical Depression Two has formed out of the strong tropical wave off the coast of Africa we've been watching, and has a good chance of becoming the Atlantic hurricane season's first named storm. Satellite loops of the storm show that heavy thunderstorm activity is increasing near the storm's center, and low-level spiral bands are getting better established. However, dry air to TD 2's north is interfering with this process, and the storm is being slow to organize. This morning's QuikSCAT pass missed TD 2.


Figure 1. Current satellite image of TD 2.

Wind shear over the storm is low, 5 knots, and is expected to remain low to moderate, 5 - 15 knots, through Thursday. Sea Surface temperatures are a marginal 26 - 27°C, and there is plenty of dry, stable air from the Saharan Air Layer (SAL) to to TD 2's north. The relatively cool SSTs and dry air mean that TD 2 will not be able to intensify quickly. However, does appear likely that TD 2 has enough going for it that it will be able to become Tropical Storm Ana later today or on Wednesday. Most of the computer models show some weak development, but none of them predict TD 2 will become a hurricane. It is unusual for storms forming this far north to make it all the way across the Atlantic to hit the Lesser Antilles Islands, and the current NHC forecast track aiming TD 2 north of the islands appears to be a good one.

Elsewhere in the Atlantic
Two other tropical waves, one passing through the Lesser Antilles Islands, and one about 600 miles east of the islands, are mentioned in NHC's Graphical Tropical Weather Outlook. Both of these waves have very limited heavy thunderstorm activity that is not increasing, and are not a threat to develop over the next two days. None of the computer models develop either of these waves.

A large, disorganized tropical wave is just leaving the coast of Africa, south of the Cape Verdes Islands. The GFS and ECMWF models continue to predict the possible development of this wave late this week.


Figure 2. Track and total rain amount from Typhoon Morakot. Image credit: NASA/GSFC.

Death toll from Typhoon Morakot in the hundreds
The death toll from Typhoon Morakot continues climb, as a landslide triggered by the storm's heavy rains hit the small town of Shiao Lin in southern Taiwan. Shiao Lin has a population of 1,300, and 400 - 600 people are missing in the wake of the landslide. Morakot killed an additional 41 elsewhere on Taiwan, with 60 missing. Earlier, the storm killed 22 in the Philippines, and went on to kill 6 in mainland China, which it hit as a tropical storm with 50 mph winds and heavy rain. Morakot's heavy rains caused an estimated $1.3 billion in damage to China.

Morakot moved very slowly as it passed over Taiwan, dumping near world-record amounts of rain. Alishan in the mountains of southern Taiwan recorded 91.98" of rain over a two-day period, one of the heaviest two-day rains in world history. The world 2-day rainfall record is 98.42", set at Reunion Island on March 15 - 17, 1952. Alishan received an astonishing 9.04 feet of rain over a 3-day period. The highest 1-day rainfall total ever recorded on Taiwan occurred Saturday at Weiliao Mountain in Pingtung County, which recorded 1.403 meters (4.6 feet or 55 inches) or rain. Nine the ten highest one-day rainfall amounts in Taiwanese history were reached on Saturday, according to the Central Weather Bureau.

Felicia continues to weaken, but is a flash flooding threat to Hawaii
Tropical Storm Felicia continues to steadily weaken, thanks to high wind shear of 30 knots. Recent satellite loops show that almost no heavy thunderstorm activity remains, and what little there is has been pushed to the northeast side of the center, exposing the surface center as a swirl of low clouds.


Figure 3. Tropical storm Felicia appeared as a swirl of low clouds with one spot of heavy thunderstorm activity to the northeast as it approached Hawaii yesterday evening.

High wind shear will continue to weaken Felicia today, and the storm is unlikely to cause major flooding problems as it moves over the islands today. The greatest danger of flooding will be over the northern islands, where Felicia's main moisture is concentrated.

Link to follow:
Wundermap for Hawaii

Blog comments not recommended
Someone managed to post a comment on my blog both Saturday and Monday containing links to a malicious web site that infected some people's computers when they clicked on the link. Our tech people have identified how to stop these attacks, but it will take some development time to engineer a fix. I recommend people not click on any links in comments posted on any wunderground blog unless you are highly confident of the destination of the link, or of the strength of your anti-virus software. I'll announce when the software fix has been made. I apologize to anybody who may have been affected by this. If you have specific information on which comments may have caused the problem, and who may be responsible, please submit a support ticket to help us out. At this time, it appears that actually reading comments on wunderground blogs is not a danger.

I'll have an update Wednesday morning.

Hurricane

Updated: 6:14 PM GMT on August 19, 2011

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Hawaii braces for Felicia; 99L near the Cape Verdes Islands may develop

By: JeffMasters, 3:55 PM GMT on August 10, 2009

A strong tropical wave (99L) is just south of the Cape Verdes Islands, 800 miles off the coast of Africa. This morning's QuikSCAT pass showed a nearly closed surface circulation, stretched out along one axis. The satellite saw winds of up to 45 mph in a band of heavy thunderstorms well south of the Cape Verdes Islands. The islands have seen winds of only 10 - 15 mph and some occasional rain showers thus far from the disturbance. Heavy thunderstorm activity associated with 99L died down this morning, but appears to be making a comeback late this morning. Wind shear is moderate, about 10 knots, and sea surface temperatures are moderately warm, about 27 - 28°C. There is a large area of dry air to 99L's north that is interfering with the storm's organization, though.


Figure 1. Current satellite image of disturbance 99L.

Wind shear is expected to be moderate, 5 - 15 knots, through Wednesday. SSTs will remain relatively constant at 27°C, but the dry, stable air of the Saharan AIr Layer (SAL) to 99L's north will be problem for it. NHC has given 99L a moderate (30 - 50% chance) of developing into a tropical depression by Wednesday morning, which is a reasonable forecast. Most of the models show some weak development, but none of them predict 99L will become a hurricane. It is too early to say if 99L will recurve north of the Lesser Antilles Islands or not, since it will be at least 5 days before the storm makes it that far. It is unusual, though, for storms forming this far north to make it all the way across the Atlantic to hit the Lesser Antilles Islands.

The GFS and ECMWF models are predicting the possible development of a new tropical wave coming off the coast of Africa late this week.

Felicia continues to weaken, but is a flash flooding threat to Hawaii
Tropical Storm Felicia has weakened steadily over the past 24 hours, thanks to cool sea surface temperatures and increasing shear. Recent satellite loops show that strong upper-level winds from the west have pushed the storm's heavy thunderstorm activity to the northeast side of the center, exposing the surface center as a swirl of low clouds. Felicia's relatively meager heavy thunderstorm activity is steadily moving away from the center of the storm.


Figure 2. History of hurricane activity over Hawaii since 1950. Hawaii islands have been hit by only 9 tropical cyclones of tropical depression or greater strength, with 4 others passing withing 75 miles of an island. Image credit: NOAA Coastal Services Center.

Sea surface temperatures (SSTs) under Felicia are 25°C, well below the 26°C threshold typically needed to sustain a hurricane. SSTs will slowly increase to 26°C by Tuesday. Wind shear has increased to a high 30 knots, and is expected to increase further to 40 knots by Tuesday. The high shear combined with the cool SSTs should continue to weaken Felicia today. I give a 30% chance that the shear will completely rip away Felicia's heavy thunderstorm activity by the time the storm reaches Hawaii, leaving only a swirl of low-level clouds that will not cause significant flooding problems. The wind speed probabilities forecast shows about a 25% chance Felicia will still be a weak tropical storm at 3 am Hawaiian time Tuesday morning, and a 15% chance the storm will have dissipated. If Felicia does hold together that long, it would be only the tenth tropical cyclone of tropical depression or higher strength to affect the islands since 1950 (Figure 2). Large swells from Felicia are already affecting the Big Island, and a high surf warning has been posted for east-facing shores of the Big Island and other Hawaiian islands. Felicia or its remnants may bring heavy rain, flash flooding, and mud slides to the islands beginning this afternoon, and a Flash Flood Watch has been posted for most of the islands.

Links to follow:
Long range radar from the Big Island
Wundermap for Hawaii

I'll have an update Tuesday morning.

Hurricane

Updated: 6:19 PM GMT on January 05, 2012

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African wave 99L may develop; Felicia threatens Hawaii; record rains from Morakot

By: JeffMasters, 3:20 PM GMT on August 09, 2009

A strong tropical wave with a high amount of spin moved off the coast of Africa yesterday, and has a good chance of becoming the first named storm of the 2009 Atlantic hurricane season. The wave was designated 99L by NHC Sunday morning. Wind shear is moderate, about 10 knots, and sea surface temperatures are moderately warm, about 27 - 28°C. There is a large area of dry air to 99L's north, but this is not currently interfering with the storm's organization. This morning's QuikSCAT pass showed a nearly closed surface circulation, stretched out along two axes. The storm can now be classified as tropical depression using the satellite-based Dvorak classification technique, but the surface circulation will have to show better definition before 99L can be classified as a tropical depression. Top winds were in the 20 - 30 mph range, as estimated by QuikSCAT and the Dvorak satellite estimates.


Figure 2. Visible satellite image of disturbance 99L.

Wind shear is expected to be low, 5 - 10 knots, tonight through Tuesday. SSTs will remain relatively constant at 27°C, so conditions favor development. NHC has given 99L a moderate (30 - 50% chance) of developing into a tropical depression by Tuesday morning, which is a reasonable forecast. The UKMET, GFS, and NOGAPS models all develop 99L into a tropical depression 1 - 4 days from now. The dry air of the Saharan Air Layer may become a problem for 99L 2 -4 days from now, as the storm moves slightly north of due west. It is too early to say if 99L will recurve north of the Lesser Antilles Islands or not, since it will be at least 6 days before the storm makes it that far.

The GFS model predicts development of a new tropical wave coming off the coast of Africa about 6 - 7 days from now.

Felicia weakens to a tropical storm, still a threat to Hawaii
Tropical Storm Felicia has weakened steadily over the past 24 hours, thanks to cool sea surface temperatures and increasing shear. Recent satellite imagery shows that wind shear is beginning to affect the storm, with strong upper-level winds from the west pushing the storm's heavy thunderstorm activity all to the east side of the center.


Figure 2. History of hurricane activity over Hawaii since 1950. Hawaii islands have been hit by only 9 tropical cyclones of tropical depression or greater strength, with 4 others passing withing 75 miles of an island. Image credit: NOAA Coastal Services Center.

Sea surface temperatures (SSTs) under Felicia are 25°C, well below the 26°C threshold typically needed to sustain a hurricane. SSTs will slowly increase to 26°C by the time the storm reaches Hawaii, though. Wind shear has increased to a moderate to high 20 knots, and is expected to increase further to 30 knots by Monday morning. The high shear combined with the cool SSTs should reduce Felicia to tropical depression strength by the time the center reaches the Hawaiian Islands Monday night. The wind speed probabilities forecast shows about a 40% chance Felicia will still be a weak tropical storm when it moves through the islands Monday night, and 40% chance it will be a tropical depression. If Felicia does hold together that long, it would be only the tenth tropical cyclone of tropical depression or higher strength to affect the islands since 1950 (Figure 1). Large swells from Felicia are already affecting the Big Island, and a high surf warning has been posted for the Big Island. Felicia or its remnants should bring heavy rain, flash flooding, and mud slides to the islands beginning Monday morning, and a Flash Flood Watch has been posted for most of the islands.

Typhoon Morakot hits China
Typhoon Morakot hit China this morning as a tropical storm, lashing the mainland with very heavy rain. Yesterday, Morakot hit Taiwan, dumping up to 80 inches of rain on the island over a 2-day period, according to news reports. The world 2-day rainfall record is 98.42", set at Reunion Island on March 15 - 17, 1952, so the Typhoon Morakot 2-day totals are some of the highest ever measured in the world. The highest 1-day rainfall total ever recorded on Taiwan occurred Saturday at Weiliao Mountain in Pingtung County, which recorded 1.403 meters (4.6 feet or 55 inches) or rain. Nine the ten highest one-day rainfall amounts in Taiwanese history were reached on Saturday, according to the Central Weather Bureau. Three are dead and 31 people missing on the island, and Morakot also killed 23 people in the Philippines.

I'll have an update on Monday.

Jeff Masters

Hurricane

Updated: 6:16 PM GMT on August 19, 2011

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Felicia not weakening; new African wave may develop

By: JeffMasters, 3:12 PM GMT on August 08, 2009

Hurricane Felicia has not weakened today, and continues to hold its own in spite of cool sea surface temperatures beneath it. Recent satellite imagery shows that the tops of thunderstorms surrounding the eye have cooled slightly, indicating that the updrafts sustaining the eyewall are maintaining their strength. The appearance of the hurricane has improved some today, with the storm assuming a more symmetric appearance.


Figure 1. Current satellite image of Hurricane Felicia.

Sea surface temperatures (SSTs) under Felicia have now fallen to 24.7°C, well below the 26°C threshold typically needed to sustain a hurricane. SSTs will slowly increase to 26°C by the time the storm reaches Hawaii, though. Wind shear is expected to remain low to moderate, 5 - 15 knots, through Monday morning. By Monday night, shear will rise to 20 knots, and remain above 20 knots thereafter. The higher shear combined with the relatively cool SSTs should mean that Felicia will be rapidly weakening in its final 24 hours before reaching Hawaii. Several of the computer models continue to show that this shear will be high enough to tear Felicia apart before it reaches Hawaii, though it appears to me that Felicia's current strength and annular structure will help it resist the shear enough to allow the storm to hit Hawaii as a tropical depression with 35 mph winds. Regardless, Felicia will bring heavy rain Hawaii beginning on Monday morning, and these rains will have the capability of causing flash floods and mudslides. The Hurricane Hunters are scheduled to make their first low-level investigation of Felicia this afternoon (around 10 am Hawaiian time). The NOAA jet also flies into Felicia today, and will drop a series of dropsondes that will be used to gather data that will be fed into today's 12Z and 00Z computer model runs.

Typhoon Morakot hits Taiwan
Typhoon Morakot hit Taiwan yesterday, battering the island with Category 1 winds and heavy rain. The Taipei airport recorded sustained winds of 56 mph, gusting to 76 mph, at the peak of the storm. Morakot killed 6 on Taiwan and 10 in the Philippines, and is headed towards a final landfall on mainland China as a tropical storm later today. Storm chaser James Reynolds intercepted the storm and posted videos on typhoonfury.com and Youtube. Morakot is still visible today on Taiwan radar.


Figure 2. Visible satellite image of a spinning tropical wave leaving the coast of Africa. The image was taken at 8am EDT 8/8/09.

African tropical wave may develop
A strong tropical wave with a moderate amount of spin is moving off the coast of Africa this morning. The wave is under about 10 - 20 knots of wind shear and has sea surface temperatures beneath it of 27°C. These conditions are probably too marginal to allow development over the next two days. As the wave moves westward away from Africa over the next few days, wind shear should slowly decrease and the SSTs will warm, potentially allowing for some slow development. The UKMET, GFS, and NOGAPS models all indicate the possibility that this will become a tropical depression 3 - 5 days from now. The ECMWF model forecasts that strong easterly winds over the wave will create too much shear to allow development. The wave is well south of the Saharan Air Layer, so dry air and African dust should not interfere over the next 3 - 5 days. I give the wave a medium (30 - 50% chance) of developing into a tropical depression in the next seven days.

I'll have an update on Sunday.

Jeff Masters

Hurricane

Updated: 6:18 PM GMT on August 19, 2011

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Hurricane Felicia weakening; Typhoon Morakot hits Taiwan

By: JeffMasters, 2:10 PM GMT on August 07, 2009

Hurricane Felicia is steadily weakening as it heads west-northwest over cooler waters. Recent satellite imagery shows that the eye is less distinct, and the tops of thunderstorms surrounding the eye are warming, indicating that the updrafts sustaining the eyewall are weakening. The storm appears rather ragged and lopsided, and probably is not as strong as the advertised Category 3 status.


Figure 1. Current satellite image of Hurricane Felicia.

Sea surface temperatures (SSTs) under Felicia have now fallen below the 26°C threshold needed to sustain a hurricane, and will continue to decline as the storm tracks west-northwest over cooler waters. By Saturday morning, SSTs should fall to 24.5°C. While wind shear is expected to remain in the low to moderate range over the next three days, 5 - 15 knots, the cooler SSTs should be able to significantly weaken the hurricane. On Sunday, Felicia will be encountering strong westerly winds aloft, which should create 15 - 20 knots of wind shear. This should be enough shear to tear the storm apart, thanks to its weakened condition due to the cool SSTs underneath it. By Monday night, when Felicia will be nearing the Hawaiian Islands, most of the computer models predict Felicia will have dissipated. One exception is the GFDL model, which predicts Felicia could be a tropical depression with 35 mph winds when it passes just south of the Big Island on Monday night. In general, the models have been trending more south with their recent runs, so it currently appears that the Big Island is most likely to feel the most impact from Felicia's rain and winds. While the current forecast calls for Felicia to have a minor impact on Hawaii, the Hurricane Hunters are scheduled to be out in full force over the next two days to monitor the storm. The NOAA jet flies into Felicia today, and will drop a series of dropsondes that will gather data to be fed into tonight's 00Z computer model runs. Regular low-level flights by the Hurricane Hunters are scheduled to begin Saturday afternoon.

Typhoon Morakot hits Taiwan
Typhoon Morakot has moved ashore over northern Taiwan this morning, and is battering the island as a strong Category 1 storm with 95 mph winds. At 9:30 pm local time today, the Taipei airport recorded sustained winds of 56 mph, gusting to 76 mph, and a pressure of 972 mb. Morakot is expected to weaken today as it interacts with the mountainous terrain of the island. Storm chaser James Reynolds is intercepting the storm and will be posting live updates on his typhoonfury.com web site and twitter for those who want to follow the storm. Morakot is also visible on Taiwan radar.


Figure 2. Radar image of Typhoon Morakot as it made landfall over northern Taiwan at 21:30 local time on 8/7/09. At the time, Taipei was experiencing sustained winds of 56 mph, gusting to 76 mph. Image credit: Taiwan Central Weather Bureau.

The Atlantic is quiet
There are no areas of disturbed weather in the Atlantic worth mentioning today, and no computer models forecast tropical storm development over the next seven days.

I'll have an update on Saturday.

Jeff Masters

Hurricane

Updated: 6:18 PM GMT on August 19, 2011

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Category 4 Hurricane Felicia peaks in intensity

By: JeffMasters, 2:51 PM GMT on August 06, 2009

Category 4 Hurricane Felicia put on a very impressive burst of intensification yesterday, peaking out with 140 mph winds. Recent infrared satellite loops show that Felicia is maintaining its Category 4 intensity, as the cloud tops surrounding the eye have stayed relatively constant in temperature.


Figure 1. Current satellite image of Hurricane Felicia.

While Felicia is an impressive hurricane now, its days are numbered. Sea surface temperatures (SSTs) under Felicia have already declined 1.5°C from yesterday, and are now 27°C--about 1°C above the 26°C threshold needed to sustain a hurricane. Felicia's west-northwest track will continue to take the storm into a region of cooler waters, which should induce a slow but steady weakening trend, beginning later today. Felicia has a very thick ring of intense thunderstorms surrounding the eye, which is characteristic of a class of hurricanes called "annular" hurricanes. Due to their structure, annular hurricane tend to resist weakening, and Felicia will probably weaken only very slowly at first. By Friday morning, SSTs should fall to 26°C, and decline to 24.5°C by Saturday. Wind shear is expected to remain in the low to moderate range over the next three days, 5 - 15 knots. On Sunday, Felicia will be encountering very strong westerly winds aloft, which should create 30 - 40 knots of wind shear. By Monday night, when most of the models predict Felicia will be nearing the Hawaiian Islands, the high shear may be able to tear the storm apart. Only the GFDL model holds on to Felicia that long, predicting that the storm will be a tropical depression with 35 mph winds when it blows through the Hawaiian Islands. The rest of the models dissipate Felicia by Monday. While the current forecast calls for Felicia to have a minor impact on Hawaii, NHC is taking this storm seriously and has scheduled a flight of the NOAA jet into Felicia on Friday. The jet will drop a series of dropsondes that will be used to gather data that will be fed into tomorrow night's 00Z computer model runs. Regular low-level flights by the Hurricane Hunters are scheduled to begin Saturday afternoon.


Figure 2. The eye of Hurricane Felicia as seen by NASA's MODIS instrument on the Terra spacecraft yesterday. The image was taken at 19 UTC 8/5/09 while Felicia was rapidly intensifying. At the time, Felica was a Category 3 hurricane with 115 mph winds. Felicia peaked in intensity as a Category 4 hurricane with 140 mph winds 9 hours after this photo was taken. Image credit: NASA.

Typhoon Morakot takes aim at Taiwan
Category 1 Typhoon Morakot is headed towards Taiwan, and is expected to make landfall tonight. Satellite loops show that the clouds surrounding the eye are developing very cold tops as they push high into the troposphere, a sign the storm is intensifying. Morakot is expected to intensify into a Category 2 typhoon before making landfall. Storm chaser James Reynolds is intercepting the storm and will be posting live updates on his typhoonfury.com web site and twitter for those who want to follow the storm. The large eye of Morakot is now visible on Taiwan radar.

The Atlantic is quiet
There are no areas of disturbed weather in the Atlantic worth mentioning today, and no computer models forecast tropical storm development over the next seven days.

I'll have an update on Friday.

Jeff Masters

Hurricane

Updated: 6:20 PM GMT on August 19, 2011

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Hurricane Felicia hits Category 3; may affect Hawaii next week

By: JeffMasters, 2:45 PM GMT on August 05, 2009

As is often the case in an El Niño year, there's nothing to talk about today in the Atlantic, but the Eastern Pacific is very active. It has been 17 years since we went this long without a tropical cyclone in the Atlantic--the hurricane season of 1992 didn't start until August 16--but in the Eastern Pacific, we've already had six named storms this year. Hurricane Felicia is the latest addition, and Felicia has put on an impressive burst of intensification this morning by powering up to Category 3 status with 115 mph winds. Recent satellite loops show that Felicia has continued to intensify, with the cloud tops surrounding the eye cooling as they push higher into the troposphere.


Figure 1. Current satellite image of Hurricane Felicia.

While Felicia is an impressive hurricane now, its days of glory will be short-lived. Felicia is currently passing over a region of warm sea surface temperatures (SSTs) of 28.5°C, a full 2.5°C above the 26°C threshold needed to sustain a hurricane. These warm waters also extend to great depth, as seen on the Ocean Heat Content image (Figure 2). Felicia's west-northwest track will take the storm into a region of cooler waters with lower Oceanic Heat Content beginning tonight, which should induce a steady weakening trend beginning Thursday night. By Friday morning, SSTs should fall to 26°C, and decline to 25°C by Saturday. While wind shear is expected to remain in the low to moderate range over the next five days, 5 - 15 knots, the cooler SSTs should be able to significantly weaken the hurricane. By Monday, when most of the computer models indicate that Felicia will be nearing the Hawaiian Islands, the storm will be at tropical depression strength with top winds of about 35 mph, according to the latest runs of the HWRF and GFDL models. Exactly how close Felicia will get to the Hawaiian Islands is a bit tricky to call right now, since the hurricane is interacting with nearby Tropical Storm Enrique. Whenever two storms get within 900 miles of each other, they tend to rotate around a common center in a dance called the Fujiwhara Effect. This sort of storm-storm interaction is a complicated affair not well-handled by the computer forecast models.


Figure 2. Total oceanic heat content (also called the Tropical Cyclone Heat Potential, TCHP) along the forecast path of Hurricane Felicia. The initial time of the forecast is 06 UTC (1 am EDT) on August 5, 2009. Oceanic heat content of 90 kJ per square cm is often associated with rapid intensification of hurricanes. Felicia is currently over waters with high heat content, but the heat content will steadily decrease over the next two days. Image credit: NOAA/RAMMB.

There are no areas of disturbed weather in the Atlantic worth mentioning today, and no computer models forecast tropical storm development over the next seven days.

I'll have an update on Thursday.

Jeff Masters

Hurricane

Updated: 6:20 PM GMT on August 19, 2011

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CSU and TSR continue to predict a near-average hurricane season

By: JeffMasters, 3:18 PM GMT on August 04, 2009

A tropical disturbance embedded in the Intertropical Convergence Zone (ITCZ), near 9N 35W, is moving west at about 15 mph. The heavy thunderstorm activity associated with this tropical wave has changed little over the past 24 hours, and remains disorganized. This morning's QuikSCAT pass showed a moderate wind shift, but nothing resembling an organized surface circulation. Top winds were in the 20 - 30 mph range. Strong easterly winds are creating about 20 knots of wind shear over the wave, which is marginally conducive for development. The disturbance is about 300 miles south of the Saharan Air Layer (SAL), so dust and dry air should not hinder development over the next few days.

Given the disturbance's current lack of organization, combined with the presence of 20 knots of wind shear, any development should be slow to occur. The forecast wind shear along the storm's path over the next five days is predicted to remain at or below 20 knots, which should allow some slow development. Sea surface temperatures (SSTs) will warm from about 28°C to 29°C as the storm progresses westward. The GFS model has been indicating some development is possible in several of its runs over the past few days, but has not been consistent with this prediction. None of the other models show any development of the system. NHC is giving the disturbance a low (less than 30% chance) of developing into a tropical depression over the next two days, which is a good forecast. The GFS and ECMWF models predict the system will be approaching the northern Lesser Antilles Islands by Sunday. Both models forecast the development of a band of very high wind shear just to the north of the islands at that time, so the long-range survival of anything that might manage to develop is in doubt.

CSU forecast team continues to predict an average hurricane season
A near-average Atlantic hurricane season is on tap for 2009, according to the seasonal hurricane forecast issued August 4 by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU). The CSU team is calling for 10 named storms, 4 hurricanes, and 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) 83% of average. Between 1950 - 2000, the average season had 10 named storms, 6 hurricanes, and 2 intense hurricanes. But since 1995, the beginning of an active hurricane period in the Atlantic, we've averaged 15 named storms, 8 hurricanes, and 4 intense hurricanes per year. The new forecast is a step down from their June forecast, which called for 11 named storms, 5 hurricanes, and 2 intense hurricanes. Their April forecast called for 12 named storms, 6 hurricanes, and 2 intense hurricanes. The new forecast calls for a near-average chance of a major hurricane hitting the U.S., both along the East Coast (27% chance, 31% chance is average) and the Gulf Coast (26% chance, 30% chance is average). The Caribbean is also forecast to have an average risk of a major hurricane (37%; 42% is average).

The forecasters noted that while sea surface temperature anomalies have increased in the tropical Atlantic and surface pressures have fallen in recent weeks, which normally would favor higher hurricane activity, the presence of El Niño conditions in the Eastern Pacific should counteract these influences. They forecast that the current weak El Niño event will strengthen to a moderate event by September:

El Niño events tend to be associated with increased levels of vertical wind shear and decreased levels of Atlantic hurricane activity. Tropical Atlantic sea surface temperatures anomalies have warmed somewhat since our early June prediction and surface pressures have fallen somewhat. But, the negative influences of El Niño-induced strong Caribbean Basin and Main Development Region vertical wind shear typically dominate over surface pressure and sea surface temperature in the tropical Atlantic.



Figure 1. Change in Sea Surface Temperature anomaly (in °C) between July 2009 and May 2009. Most of the tropical Atlantic has warmed, relative to normal, over the past 2 months. Image credit: NOAA/ESRL.

Analogue years
The CSU team picked four previous years when atmospheric and oceanic conditions were similar to what we are seeing this year: weak to moderate El Niño conditions, and average tropical Atlantic and far northern Atlantic SSTs. Those four years were 2002, which featured Hurricane Lili that hit Louisiana as a Category 1 storm; 1965, which had Category 3 Betsy that hit New Orleans; 1963, which had Category 4 Hurricane Flora that devastated Cuba; and 1957, which didn't have any hurricanes that hit hit land during the peak part of hurricane season. The mean activity for these four years was 9 named storms, 4 hurricanes, and 2 intense hurricanes--almost the same as the 2009 CSU forecast.

How accurate are the August forecasts?
The August forecasts by the CSU team have historically offered a skill of 45 -62% higher than a "no-skill" forecast using climatology (Figure 2). However, they are using a new forecast scheme this year, so it is difficult to judge how skillful this year's forecast might be.


Figure 2. Accuracy of long-range forecasts of Atlantic hurricane season activity performed at Colorado State University (CSU) by Dr. Bill Gray's team (colored squares) and Tropical Storm Risk, Inc. (TSR, colored lines). The skill is measured by the Mean Square Skill Score (MSSS), which looks at the error and squares it, then compares the percent improvement the forecast has over a climatological forecast of 10 named storms, 6 hurricanes, and 2 intense hurricanes. TS=Tropical Storms, H=Hurricanes, IH=Intense Hurricanes, ACE=Accumulated Cyclone Energy, NTC=Net Tropical Cyclone Activity. Image credit: TSR.

August 2009 Atlantic hurricane season forecast from Tropical Storm Risk, Inc.
The British private forecasting firm Tropical Storm Risk, Inc. (TSR) also issued a new forecast today, and have increased their numbers by 20% from their June and July forecasts. TSR is also calling for a near-average season, predicting 12.6 named storms, 6.5 hurricanes, 2.8 intense hurricanes, and an Accumulated Cyclone Energy (ACE) 103% of average. Their June forecast called for 10.9 named storms, 5.2 hurricanes, 2.2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) 72% of average. The storm numbers are slightly above the 50-year average of 10 named storms, 6 hurricanes, and 2 intense hurricanes. TSR predicts a 40% chance of an above-average season, 44% chance of a near-average season, and a 19% chance of a below-average season, as defined by ACE index. TSR rates their skill level as 51% above chance at forecasting the number of named storms, 60% skill for hurricanes, and 44% skill for intense hurricanes. These are far higher skill numbers than the June ones: 26% above chance at forecasting the number of named storms, 15% skill for hurricanes, and 19% skill for intense hurricanes.

TSR projects that 3.8 named storms will hit the U.S., with 1.6 of these being hurricanes. The averages from the 1950-2008 climatology are 3.2 named storms and 1.5 hurricanes. Their skill in making these August forecasts for U.S. landfalls is 25% above chance. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.1 named storms, 0.5 of these being hurricanes. Climatology is 1.1 named storms and 0.5 hurricanes.

TSR cites one main factor for their increased forecast: higher sea surface temperatures than expected over the tropical Atlantic, due to the fact that the trade winds over the Atlantic should be slower than originally anticipated. Faster than average trade winds create less spin for developing storms, and allow the oceans to cool down, due to increased mixing of cold water from the depths and enhanced evaporational cooling.

The CSU and TSR groups are done making forecasts for the coming hurricane season, but NOAA is still due to put out an August update.

I'll have an update on Wednesday.
Jeff Masters

Hurricane

Updated: 6:20 PM GMT on August 19, 2011

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World storm surge records

By: JeffMasters, 2:41 PM GMT on August 03, 2009


The Bathurst Bay Cyclone, also known as Tropical Cyclone Mahina, which struck Bathurst Bay, Australia on March 5, 1899, is generally credited with the world record for storm surge. The cyclone's storm surge is variously listed at 13 - 14.6 meters (43 - 48 feet). The Category 5 cyclone was a monster--with sustained winds in excess of 175 mph and a central pressure between 880 and 914 mb. Mahina killed at least 307 people, mostly on pearling ships, and was the deadliest cyclone in Australian history. The eyewitness account of Mahina's record storm surge was provided by Constable J. M. Kenny, who journeyed to Barrow Point on Bathurst Bay to investigate a crime on the day of the storm. While camped on a ridge 40 feet above sea level and 1/2 mile inland, Kenny's camp was inundated by a storm wave, reaching waist-deep. On nearby Flinders Island, fish and dolphins were found on top of 15 meter (49 foot) cliffs. However, an analysis by Nott and Hayne (2000) found no evidence of storm-deposited debris higher than 3 - 5 meters above mean sea level in the region. They also cited two computer storm surge simulations of the cyclone that were unable to generate a surge higher than three meters. Indeed, Bathurst Bay is not ideally situated to receive high storm surges. The Great Barrier Reef lies just 20 - 40 km offshore, and the ocean bottom near the bay is not shallow, but steeply sloped. Both of these factors should conspire to keep storm surges well below the record 13 - 14.6 meters reported. The authors concluded that the actual surge from the Bathurst Bay Cyclone may have been 3 - 5 meters. The observed inundation at 13 meters elevation, plus the observation of dolphins deposited at 15 meters above sea level, could have been caused by high waves on top of the surge, they argue. Waves on top of the surge (called "wave run-up") can reach five times the wave height at the shore for steeply fronted coasts like at Bathurst Bay. Since waves in the Bathurst Bay Cyclone could easily have been 3 meters, 15 meters of wave run-up on top of the surge is quite feasible. Since wave run-up doesn't count as surge, the status of the 1899 Bathurst Bay Hurricane as the world-record holder for storm surge is questionable. However, the event is certainly the record holder for the high water mark set by a tropical cyclone's storm surge, an important category in its own right.


Figure 1. Satellite image of Bathurst Bay, Queensland Province, Australia. The record 43 - 48 foot storm surge wave occurred on Barrow Point, marked by an "x" on the map above. Image credit: NASA.


Figure 2. Track of the 1899 Bathurst Bay cyclone. Bathurst Bay is located at the point where the 914 mb pressure is listed. Image credit: Whittingham, 1958.

Australian storm surge records
The largest storm surges in Australia occur in Gulf of Carpentaria, due to the large expanse of shallow water there (the Gulf of Carpentaria is the large bay to the left of the zoomed-in map of Bathurst Bay shown above). According to an email I received from Australian hurricane scientist Jeffrey Callaghan, "From all reports the storm surge from the disastrous 5 March 1887 cyclone flooded almost all of Burketown (some 30km inland from the Gulf). A copy of a 1918 report to the Queensland Parliament from the Department of Harbours and Rivers Engineer refers to the sea rising to 5.5 metres above the highest spring tide level at the Albert River Heads. This level is about 8 metres (26.2 feet) above Australian Height Datum (AHD). The biggest measured surge in the Gulf of Carpenteria occurred on 30 March 1923, when a surge of 21.4 feet was recorded at a Groote Eylandt Mission".

So what is the world storm surge record if the Bathurst Bay cyclone does not qualify? Well, I haven't researched storms in the Indian Ocean or Pacific Typhoons yet, but it might be difficult to find any storm that beats Hurricane Katrina's 27.8 foot storm surge.

References:
Nott, J, N. Hayne, 2000: How high was the storm surge from Tropical Cyclone Mahina?", Australian Journal of Emergency Management, Autumn 2000.

Anonymous, 1899, The Outridge Report--The Pearling Disaster 1899: A Memorial", The Outridge Company, 1899

Whittingham, 1958, "The Bathurst Bay hurricane and associated storm surge", Australian Meteorological Magazine, No. 27, pp. 40-41. Scanned and put on-line courtesy of John McBride.

I'll have an update on Tuesday, when the latest CSU seasonal hurricane forecast comes out at 11am EDT.
.
Jeff Masters

Hurricane

Updated: 7:47 PM GMT on April 04, 2014

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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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