Today is winter solstice--the darkest day of the year--and an appropriate time to revisit America's other darkest day of the year, August 29. The National Hurricane Center (NHC) issued its official Tropical Cyclone Report for Katrina on Tuesday. Katrina officially made landfall as a Category 3 hurricane, not a Category 4. Ground-based and aircraft measurements only support 110 knot winds (127 mph) at Katrina's first landfall near Buras, Louisiana. Katrina weakened only slightly before her second landfall, and was still a Category 3 hurricane with 105 knot (121 mph) winds on the Mississippi coast. The NHC report also stated that the highest sustained winds over metropolitan New Orleans were only of Category 1 or 2 strength, although buildings over 25 stories high may have seen winds a full category higher.
The reason Katrina was originally classified as a Category 4 at landfall was because winds measured by the Hurricane Hunters at flight level (10,000 feet) were 150-155 mph. The normal rule of thumb used to estimate surface winds is a 10% redution from the winds at 10,000 feet. This rule of thumb was applied for the official NHC advisories issued at the time of Katrina's landfall, and made Katrina a Category 4 hurricane with 135-140 mph surface winds. However, detailed analysis of the wind structure of Katrina in data gathered by Doppler radar and dropsondes showed that at landfall, Katrina had its highest winds in an unusually strong band of winds between 2 and 4 km (the flight level of the Hurricane Hunters was about 3 km). Normally, the highest winds in a hurricane are found much lower, near .5 - 2 km. Surface winds measured by dropsondes, surface towers, and the SFMR microwave radiometer on the aircraft all agreed that the surface winds at landfall were no higher than 100 knots (115 mph). NHC adjusted these upwards by 10% to account for the fact that the strongest winds were likely not sampled. The 10% adjustment left Katrina just 5 mph shy of Category 4 status--but still a very potent and deadly major Category 3 hurricane.
The 10% reduction "rule of thumb" was not valid for Katrina at landfall, probably because the storm's convection was weakening at that time. Because momentum transport from aloft to the surface was impaired by the weakening convention, Katrina was less able to carry the strong winds that were aloft down to the surface. Thus, winds at the surface were about 80% of the winds measured at 10,000 feet. Still, NHC does mention that given the uncertainties and large wind field of Katrina, the very tip of the Mississippi Delta near Buras may have received Category 4 winds for a few minutes, and it is possible Katrina really was a Cat 4 at landfall.
Why Katrina weakened at landfall
At peak intensity, Katrina was a Category 5 hurricane with 150 kt (174 mph) winds, but in the 18 hours before landfall weakened to 110 kt (121 mph). This weakening occured as a result of entrainment of dry air into the storm, slightly cooler sea surface temperatures near the coast, and interaction with the land. NHC notes that the relative importance of these three factors cannot be determined without a lot more study, but all 11 hurricanes with pressures less than 973 mb that have hit the Gulf coast the past 20 years have weakened in the 12 hours prior to landfall. Thus, Katrina's weakening should come as no surprise. Note, however, that Hurricane Camille of 1969 did not weaken when it pounded Mississippi as a Category 5 hurricane; perhaps it's small size protected it from substantial land interaction and entrainment of dry air.
Storm surge
To me, the biggest disappointment in the report came in the treatment of Katrina's storm surge. No storm surge data was presented for New Orleans. No mention was made that Katrina, despite its Category 3 strength at landfall, pushed a Category 5 level storm surge to the coast. The report noted that official storm surge measurements were unavailable, due to failure of most of the tide gauges. However, one unofficial storm surge height of 27 feet at the Hancock County Emergency Management Office in Mississippi was mentioned, which would make Katrina's storm surge the highest on record for an Atlantic hurricane. The previous record was Hurricane Camille's 24.7 feet. Any surge above 18 feet is considered a Category 5 level storm surge. I've seen unofficial estimates that the storm surge affecting the eastern side of New Orleans was 18-25 feet high, which is clearly a Category 5 storm surge. Not surprisingly, the levees protecting the east side of the city were overwhelmed and failed in multiple locations. However, observational data and computer modeling indicate that storm surge entering the canals from Lake Pontchartrain reached 9 to 11 feet in the 17th Street Canal and 11 to 12 feet in the London Avenue Canal. The flood walls were 13.5 feet high or higher along much of the two canals and were designed to withstand water rising to 11.5 feet. A Category 3 storm surge is 9-12 feet, so these flood walls failed in a Category 3 level storm surge, even though they were supposedly designed to withstand that type of storm surge.
Pressure
Katrina officially made landfall at Buras, LA, with a central pressure of 920 mb. This is the third lowest pressure on record for a U.S. landfalling hurricane, surpassed only by the two Category 5 hurricane to hit the U.S.--the Florida Keys Labor Day Storm of 1935 (892 mb) and Hurricane Camille of 1969 (909 mb). Katrina had the lowest pressure ever measured for a Category 3 hurricane; the previous record was 930 mb for Hurricane Floyd of 1999. Katrina's unusually low winds were primarily due to the fact that Katrina was a huge storm--the change of pressure from outside the storm to inside the storm happened over a large distance. It's the pressure gradient--the change of pressure with distance--that drives winds, not the pressure itself.
Death Toll
The official death toll so far is 1336, with 1090 of those victims in Louisiana and 228 in Mississippi. This makes Katrina at least the fifth deadliest U.S. hurricane of all time. The death toll could go much higher, making Katrina the third deadliest. Over 4,000 people are still listed as missing. Most of these missing people are probably alive and well, according to Kym Pasqualini, CEO of National Center for Missing Adults. However, she indicates that 1,300 of the missing from the most heavily damaged areas of New Orleans are a matter of great concern, and many of these people may have died in the storm.
Damage
The report quotes a preliminary figure of $75 billion in damage for Katrina, a number used by the American Insurance Services Group (AISG). This would make Katrina, by a least a factor of two, the costliest hurricane ever. A recent estimate by the world's largest re-insurance company, the Swiss Munich Re Foundation, put Katrina's total damage closer to $125 billion.
Forecast accuracy
NHC gives themselves high marks for forecast accuracy for the 2 1/2 days prior to Katrina's landfall. Indeed, their landfall location forecasts had errors more than a factor of two better than average. These exceptionally accurate forecasts likely saved hundreds of lives. On the other hand, NHC intensity forecasts for Katrina were up to a factor of two worse than average, and perhaps more lives could have been saved had these intensity forecasts been better.
Jeff Masters
1st. Baptist Church
Gulfport, MS
I work for Louisiana Wildlife and Fisheries. We are in search and rescue mode. Storm damaged and flooded house.
Page: 1 | 2 | 3 | 4 — Blog Index
We require an explanation, given in a sound, scientific manner, and supported by verifiable, objective evidence, regarding the following two things:
1. How you arrived at your final landfall wind speeds at Buras, LA and the MS Gulf Coast, when numerous measurements from inland stations at different points along the storms path clearly indicate much higher wind speeds at landfall.
2. How a supposed "Category 3" hurricane, with a landfall strength and windfield size comparable to Hurricane Ivan of last year, carried a surge almost twice the height of Ivan's into the Gulf Coast.
Sincerely,
TampaSteve
" think after seeing these devestating hurricanes the past two years all of us have to realize there isn't an "exact" to what each catagroy a storm can do. In Punta Gorda wheere I live on the water a "normal high end Cat 4" storm we would all be under water from the surge. The highest elevation home is at 10 feet. But as you know with Charley due to its size, fast moving and strengthening ect.. we didn't get the deadly surge just the wind damage. But now only what 18 short months later the majority of the clueless in town say oh well we lived through Charley and look we are still here. I always voice my opinion saying yes but remember if it had been a "normal Cat 4 the whole county would have been washed away. And they still don't get it.... Hello???"
AMEN, brutha! My in-laws live in PGI...I agree 100%!
Hope everyone is haveing a good day. For Southern California a large high sits over the area like a huge brick. In Southern California my christmas tempture will be in the 80's. Thats not holiday like :-(. I remember last year winter kicked into high gear the day after christmas where we had on and off rain from the 26 of December tell January 10th if my memory is correct. I also beleave that the city of LA got the second most amount of rain ever in a 24 hour period someing like 5.30 inces. For LA that is something I would probally never see again if I lived around the area for the rest of my life.
This year however I have already wrote off this winter as below normal and am looking toward next year.
Tampa Steve
I live in PGI too. All of PGI would be gone if Charley had the "normal" Cat 4 surge. I might not be here today. We rode out the storm in the closet with life jackets, pool floats and an ax to take to the attic if needed. God, what a day I will never forget. I will definitely not stick around for anaother and risk it ever again. I have had motel reservations made for every storm after Charley and will do the same next year.Where do your in laws live? We are renting a place over here and moved in two monthes before Charley roared through. Oh by the way I am a sista LOL
Bill
www.guide2cozumel.com
Notes on Katrina's surge levels
Not all of the information on Katrina's surge along the MS Gulf Coast has been made available. The initial NHC Katrina report did not go into extensive detail about the surge.
High water marks do not by themselves denote surge levels. High water marks, especially outdoor ones, also include height from waves. Waves right at the shoreline can be quite high. Allegedly a DVD with amateur video which was distributed locally in limited quantities in MS, showed approximately 30 foot waves at the shore in Gulfport. Wave estimates at the shore there were already in that range; waves on top of the 20+ foot surge there were large enough to wash all the dolphins out of their 30 foot high tank at the marina. However wave action, even as small as half a foot, can travel very far inland. Tides are also a factor in computing the actual surge. Tides along the MS Gulf Coast are not a significant factor; the different between high and low tides is only a couple of feet at most. This factor is easy to take into account as the times and heights of high and low tide are known. These factors must be subtracted to obtain the water height generated from the hurricane surge.
Just as with sustained wind values, identifying the highest level of surge is not the same thing as saying that level of surge occurred everywhere. It is important to quantify the extent of the area flooded by each category of surge.
We can make an unofficial but informed guess as to what areas of the MS coast received what categories of surge, based on information currently available, with the caveat that it may be changed later, and we are not 100% certain of its accuracy.
What tools are currently available on the internet to determine this information?
First, FEMA has published maps of the surge on the MS coastline. These maps show an extremely limited number of high water marks, mostly outside, and that information is too sparse right now to give an overall picture of the surge. Along with these selected high-water marks (which the NHC Katrina report may possibly have implied are not valid because the wave and tide levels have not been subtracted yet), the FEMA maps show all areas which were flooded, by some level of water, by the surge. This may mean only one foot of water from surge (as in the parking lot of the Jackson County sub station in Ocean Springs), to 25 feet of water from surge (as along the southern coast of the Waveland / Bay St. Louis area).
http://www.fema.gov/hazards/floods/recoverydata/katrina_ms_index.shtm
Second, we can use detailed online elevation maps such as Topozone, to identify the elevation at any location along the coast.
http://www.topozone.com
Third, we can use the Hurricane Evacuation Study (HES) maps for Mississippi, from the USACE web site. These maps were generated from SLOSH runs (the parameters for the runs are documented as well), and seem to be extraordinarily accurate. These maps show what areas of each county, down to individual addresses, would be flooded for each of the five hurricane category classifications. The HES maps do not show a range of heights associated with each hurricane category, and there is a good reason for that: the height differs with each location based on surrounding topography.
http://ekman.csc.noaa.gov/website/FEMA_EvacZ/AddressPage.htm?state=mississippi
By comparing the elevation maps to the HES maps, we can determine at what level each category of surge begins, based on the location. Thus, Category 4 surge begins at 21 feet for BSL, Pass Christian, and Ocean Springs, but at only 15 feet for Pascagoula. This last may have been an error on the part of the model, underestimating the vulnerability of this location, because these Cat 4 levels on the map were flooded, or there may have been certain characteristics of the surge that resulted in an anomaly with higher surge here, because the flooding of the Pascagoula River Basin in general was extreme (
http://www.nasa.gov/vision/earth/lookingatearth/h2005_katrina.html ).
Fourth, FEMA has also provided an overview map with their estimates on the height of the surge across the MS coastal region.
http://www.fema.gov/hazards/floods/recoverydata/pdf/ms_overview.pdf
Finally, to back up the FEMA flood maps, we can go directly to the aerial images from NOAA taken shortly after the event (but of course this would be after most of the floodwater had receeded). It cannot always be seen from these maps which areas were flooded. Some places, such as in a large part of Hancock County, everything is covered with mud, but other places will have not discernable sign, and you cannot always go by the debris line as an indicator either.
http://ngs.woc.noaa.gov/katrina/
Based on this information, and assuming it to be accurate, we can answer the following questions:
What areas of the MS coast experienced Cat 3 level surge from Katrina? What areas, if any, had Cat 4 level surge? Cat 5? Where did this occur, and how large were these areas?
This is not completely straightforward. It is clear that the highest level of surge did not go very far inland. All of the areas that experienced the highest levels of surge were right on the shore, and most are so low they already flood at Cat 1 level. For those we would want to rely on accurate high-water marks to determine what level surge occurred. But that information is not available right now. Luckily we can go by the FEMA overview map to determine where these areas are. Secondly we can look at the areas along the coastline that were at an elevation of Cat 4 or Cat 5, and see if any of these areas flooded. The problem with this method is that there is very little actual real estate along the entire coastline that is that high, as the MS coast is one of many areas in the US that is a flood plain. Also, these areas may not all be right on the coast.
In addition, the FEMA overview map only lists the surge in terms of feet. How can we determine what level of surge that corresponds to? Considering it can be different depending on location, we have to first use the elevation maps in conjunction with the HES maps to find out where Cat 3, Cat 4, and Cat 5 surges begin.
Before we get into this level of detail, we find that at a high level, comparing the FEMA overview map to the HES map, solid Cat 3 surge occurred along the entire MS coastline. This Cat 3 surge finally ended, inland, just NE of Pascagoula City limits, at around 611 and Old Mobile Hwy. The extensive amount of land to the south of that location, all the way into AL and to Mobile Bay, was at Cat 1 and 2 level, and was all flooded. So the Cat 3 surge ended inland, just miles short of the AL border. Cat 1 and 2 surge extended eastward into Alabama. On the western edge of the MS coast, in Hancock County, the surge ended just south of the NASA Stennis facility on the MS / LA border, and just north of Kiln and the Diamondhead areas. The most extensive area of inland surge was on the eastern edge of the MS coastline, along the Pascagoula River Basin, and this is predicted on the MS HES maps.
This, just by itself, is really the most remarkable thing about Katrina's surge: the extent of the Cat 3 surge, not the small area that may have been affected by higher Cat 4 or 5 surge.
Did any of these areas at higher surge levels exist?
We know from the overview map that the highest levels of surge, in feet, were along the southern shore of the Waveland / Bay St Louis area, the Diamondhead bay shore area south of I-10, and an area of the coast from about Menge Ave in the Pass, east to the Long Beach / Gulfport border, so we can decide that the coast on either side of Saint Louis Bay is a good place to start identifying the highest level of surge.
Also, we can use the MS HES maps to locate any land that is at Cat 4 or Cat 5 surge level in Hancock and western Harrison Counties. We find that there are very few areas and that they are very small. Just south of the BSL bridge we find the eastern end of four small city blocks that are at Cat 4 level right along the shoreline. Another area of land that will not flood until Cat 4 or Cat 5 level is SW of there, and runs inland, parallel to the southern shoreline, cutting across the man-made lake for the water treatment plant (an easily-found location on a map). The remainder of the Hancock County shoreline, and quite a ways inland, will flood with a Cat 3. In western Harrison County, along the shoreline, we find only one narrow area, starting just west of Menge Ave in Pass Christian, that runs east along the shoreline into Long Beach, Gulfport, and Biloxi, that will only flood at Cat 4 or Cat 5 level.
Locating these areas on topozone.com, and following the brown elevation lines (which are given every five feet, for instance 15, 20, and 25 foot elevations), and comparing with the MS HES maps, we can make a general assessment at what elevation Cat 4 and 5 start for these particular locations. We find that it looks like Cat 4 starts at around 21 feet in elevation in both places, and Cat 5 at 24 feet for BSL, and 25 feet for the Harrison County area previously mentioned. This is a guess, but is probably not off by more than a foot.
Now, we can use the FEMA flood maps to see how much of each of these areas did actually flood.
In BSL, we find that everything was under water except for most (but not all) of the areas that only flood with a Cat 5 surge. We find that the 25 foot surge identified on the FEMA overview map did not reach far enough inland to affect any areas that were not already flooded. We find that all the tiny areas of land at Cat 4 did flood, and the edges of the Cat 5 area around the water treatment plant flooded. So we know that the highest surge in this area was approximately 24 to 25 feet. Perhaps the area on the Waveland / BSL coastline received higher surge, even though they were already well under water, but we will have to wait for accurate high water marks to assess that.
In eastern Pass Christian, we find that the areas that are at Cat 4 level flooded, but the strip of land at Cat 5 level (beginning at 25 feet of elevation) did not. Since this area is right on the shoreline, we can conclude the surge there reached 25 feet but did not go much over that, if any, and most likely did not reach Cat 5 level.
We can conclude that very small areas of the MS coastline, most likely less than one percent of the land on the MS coast that was flooded directly from surge, were subjected to a surge that just barely edged over the Cat 5 level, although the total area receiving surge at the Cat 5 level was so small compared to the total area of land inundated by surge, that it is almost negligible, statistically speaking. Most of the area receiving a Cat 5 level surge was already so low that it was well underwater, the difference in the height of the surge was probably not a significant factor in incurring the total property devastation, but, if it can be quantified, will be useful for denoting the record height of the surge. Another small area, bordering that, perhaps a couple percent of the total area, received Cat 4 surge. The areas that were on the shore were also devastated by waves on top of the surge.
The majority of the coastline received Cat 3 surge, and the area receiving this level of surge covered a wide range that was unprecedented, spanning the entire MS coastline, and reaching inland a considerable distance along all but a few miles close to the AL border. The area receiving Cat 1 and 2 level of surge was even more extensive, reaching completely to the northern Jackson County border, in eastern MS, and into AL, crossing Mobile Bay.
MEOW - maximum envelope of wind
MOM - maximum of MEOWs
I didn't write those last paragraphs very clearly. Here is a rewrite to sum up the areas with Cat 4 and 5 level surge:
We can conclude that one very small area of the MS coastline, the immediate coast of Waveland and southern BSL, was subjected to a surge that just barely edged over the Cat 5 level. This area was so small compared to the total area of land inundated by surge, that it is almost negligible, statistically speaking. The elevation of this area was already so low that it was well underwater, the difference in the height of the surge was probably not a significant factor in incurring the total property devastation, but, if it can be quantified, will be useful for denoting the record height of the surge.
The areas that received Cat 4 surge were Waveland and the part of BSL that curves into the bay and GOM, and a small portion of coastline along eastern Pass Christian, to the Long Beach / Gulfport border, and Diamondhead area south of I-10.
so good night all
I have just created a new blog relating to global warming and whose responsible for it. I believe everyone will at least find it interesting reading whether they like the contents of it or not.
Here is the link to my blog.:)
Link
Your friend,
Tony
I have only a couple comments. First, I read the NHC report before coming here, and I found it interesting that at least a couple of the unofficial reports of wind speeds, especially in FL, were taken from wunderground.com. I guess it is fair to say that NHC knows we are out here and have at least some respect for what members are doing .
The other thing that struck me is that consistently NHC is unable to report accurately regarding wind speed, wave / surge heights, etc. because THE MEASURING EQUIPMENT FAILED!!!!! If anybody is writing to the NHC, someone should be querying that little fact, with a view to finding out what is to be done about it. I think it is ridiculous that more than 100 years after Galveston there are still no accurate, DURABLE measuring tools on the ground.
Rather than disputing over the accuracy of landfall intensities, we all should be encouraging the "powers that be" to actively seek avenues whereby the quality of reporting in future storm events can be improved.
Personally I believe the storm was a cat 4 at landfall, it's satellite presentation was more consistent with a 4. However we cannot know for certain... even at Cat 3 speeds winds instruments are destroyed and operational data is sparse and often inaccurate.
But that's not the point. This has nothing to do with data or analysis... this is a purely political move probably motivated by pressure from certain factions within the government. There is a huge drive to blame Bush and the Army Corps of Engineers for this disaster. This drive is especially strong in academic sections of the government... in other words, including the NHC. By downgrading to a Cat 3, regardless of whether or not it is true, the NHC creates ammunition for the media to put pressure on the federal government to pay for the disaster.
This is the real reason, and let's not kid ourselves by saying it has anything to do with the data. The NHC, along with some other organizations within the government, want to put pressure on the Bush administration. By blaming the Army Corps of Engineers for the levee failure and Bush for not spending federal money on them, the NHC along with the LA government and many others want to create a situation where Bush and the federal government must pay massive compensation to the victims.
I've heard a lot of arguing about wind speeds and storm surge, etc... but truth be told, Cat 3 or 4.. it doesn't matter. The storm was massive, powerful, carried a huge surge and destroyed New Orleans along with whole sections of Mississippi. We already know this, and political stupidity aside it doesn't much matter what us mere mortals rate a disaster. It is what is.
Viewing: 151 - 176
Page: 1 | 2 | 3 | 4 — Blog Index