Remainder of July hurricane outlook
Not much has changed in the Atlantic since my early July Atlantic hurricane outlook posted two weeks ago. Tropical cyclone activity typically picks up a bit during the last half of July, but we are still a month away from when hurricane season really gets going. Since the current active hurricane period began in 1995, nine of 14 years (63%) have had a named storm form during the last half of July. We had two last-half-of-July named storms last year--Christobal and Dolly. As seen in Figure 1, most of the late July activity occurs in the Gulf of Mexico, Western Caribbean, and Carolina waters. However, a few long-track "Cape Verdes" hurricanes begin to occur. These are spawned by tropical waves that come off the coast of Africa. Tropical waves serve as the instigators of about 85% of all major hurricanes.
Figure 1. Tracks of all tropical storms and hurricanes 1851 - 2006 that formed July 16 - 31.The Gulf of Mexico coast is the preferred strike location. There are still very few major Cape Verdes-type hurricanes forming in the last half of July.
Sea Surface Temperatures
Sea Surface Temperature (SST) anomalies have warmed slightly over the past two weeks, and are about 0.3°C (0.5°F) above average over the tropical Atlantic between Africa and Central America (Figure 2). These are some of the coolest SST anomalies for this time of year that we've seen since 1994. The strength of the Azores-Bermuda high has been near or slightly below average over the past two weeks, driving slightly below average trade winds. Weaker trade winds don't mix up as much cold water from the depths, and cause less evaporative cooling. The latest 2-week run of the GFS model predicts continued near-average or slightly below average-strength trade winds through the end of July, so SSTs should remain slightly above average during this period.
Figure 2. Sea Surface Temperature (SST) departure from average for July 13, 2009. SSTs were about 0.3°C (0.5°F) average over the tropical Atlantic's Main Development region for hurricanes, from Africa to Central America between 10° and 20° North Latitude. Note the large region of above average SSTs along the Equatorial Pacific off the coast of South America, the hallmark of a developing El Niño episode. Image credit: NOAA/NESDIS
El Niño
El Niño conditions continue to amplify over the tropical Eastern Pacific. Ocean temperatures in the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region", are now 0.4°C above the threshold for a weak El Niño, according to the Australian Bureau of Meteorology (Figure 3). An increase of another 0.1°C will push the current El Niño into the "moderate" category. NOAA's Climate Prediction Center issued an El Niño Advisory earlier this month. The latest set of mid-June runs of the El Niño computer models are almost universally calling for El Niño conditions to become well-established for the peak months of hurricane season, August - October. It is likely that Atlantic hurricane activity will be suppressed in 2009 due to the strong upper-level winds and resulting wind shear an El Niño event usually brings to the tropical Atlantic.
Figure 3. Sea Surface Temperature (SST) departure from average for the the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region"). El Niño conditions exist when the SST in this region rises 0.5°C above average. As of July 8, 2009, SSTs in the Niño 3.4 region had risen to 0.9°C above average. To be considered an "El Niño episode", El Niño conditions must occur for five consecutive months, using 3-month averages. Image credit: Australian Bureau of Meteorology.
Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream's band of strong high-altitude winds is the main source of wind shear in July over the Atlantic hurricane breeding grounds, since the jet is very active and located quite far south this time of year.
The jet stream over the past three months has been locked into a pattern where a southern branch (the subtropical jet stream) brings high wind shear over the Caribbean, and a northern branch (the polar jet stream) brings high wind shear offshore of New England.
The jet stream is forecast to maintain this two-branch pattern for the next week. However, during the final week of July, the subtropical jet is forecast to weaken. This will leave regions of low wind shear over the Caribbean and Gulf of Mexico for the final week of July (Figure 4), increasing the chances of hurricane development.
Figure 4. Wind shear in m/s between 200 mb and 850 mb on July 31, 2009, as forecast by the 00Z July 15, 2009 run of the GFS model. The subtropical jet is forecast to weaken by this time, leaving regions of low wind shear over the Caribbean and Gulf of Mexico for the final week of July. Wind speeds are given in m/s; multiply by two to get a rough conversion to knots. Thus, the red regions of low shear range from 0 - 16 knots.
Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa, and the Saharan dust storms have been quite active over the past month. Expect dust from Africa to be a major deterrent to any storms that try to form between Africa and the Lesser Antilles Islands in July. Several well-developed African waves have been done in by dry air from Africa over the past few weeks.
Steering currents
The steering current pattern over the past few weeks has not changed much. A persistent trough of low pressure has remained entrenched over the Eastern U.S. all summer, bringing cool and relatively moist weather to the eastern half of the country. This trough is strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are predictable only about 3 - 5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. At present, it appears that the coming two weeks will maintain the strong trough over the Eastern U.S., which decreases the hurricane risk to the U.S. Gulf Coast. There is no telling what might happen to the steering current pattern during the peak months of August, September, and October, but it is often difficult to break a months-long steering current pattern like the current one.
Summary
Recent history suggests a 63% chance of a named storm occurring in the last half of July. Given that none of the computer models are forecasting tropical storm formation in the coming seven days, and SST and wind shear patterns look pretty average, I'll go with a 30% chance of a named storm forming this month. Such a storm would most likely form near the end of the month, when wind shear is expected to decline due to a weakening of the subtropical jet stream. The last time we went this long in the season without a named storm forming was in 2004, when the first storm (Alex) formed on August 1.
I'll have a new post on Friday.
Jeff Masters
Reader Comments
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:)
Looks like Texas is finally getting an eensy weensy bit of rain.
Link
;)
As I always say, the tropical wave will be there, convection or no convection. There is still a bit of dust to disperse and vertical shear is still questionable. SSTs however, become progressively warmer as one head west, and it has able to produce some level of convection in the EATL. It also has a noticable mid-low level circulation. 30% for now.
Tropical Storm "ISANG" continues to threaten Extreme Northern Luzon.
Tropical Cyclone Bulletin #9
===================================
At 5:00 AM PhST, Tropical Storm Isang (MOLAVE) located at 18.0°N 123.2°E or 150 km east southeast of Aparri, Cagayan has 10 minute sustained winds of 85 km/h (45 knots) with gustiness of 100 km/h (55 knots).
Warning Signals
=================
Signal Warning #2 (60-100 kph winds)
Luzon Region
-----------
1.Batanes
2.Cagayan
3.Babuyan Group
4.Calayan Group
5.Isabela
6.Kalinga
7.Apayao
8.Abra
9.Ilocos Norte
Signal Warning #1 (30-60 kph winds)
Luzon Region
------------
1.Northern Aurora
2.Quirino
3.Nueva Vizcaya
4.Ifugao
5.Benguet
6.Mt. Province
7.Pangasinan
8.La Union
9.Ilocos Sur
Additional Information
======================
Residents living in low-lying areas and near mountain slopes under storm warning signals #1 & #2 are alerted against possible flashfloods and landslides.
Those living along the coast under signal #2 are advised to be on alert against big waves generated by the storm.
The public and the disaster coordinating councils concerned are advised to take appropriate actions and watch for the next bulletin to be issued at 11 AM today.
Since it is already at 12n, if it is to develop further, it will have to traverse and survive a pool of slightly cooler SSTS around 79 - 81F between 35W and 45W (which is where the previous wave choked). Once it gets to 50W intact .... look out.
And the wave approaching 50W has a fair bit of turning associated with it too. I believe the convection will start to increase with it from tonight now that it is moving over slighly warmers waters, better low level convergence and that the dust outbreak in the vicinity has tapered off.
DMAX should be starting later tonight... but not quite yet. :)
Yes. Just been keeping an eye on it, but nothing to get excited or concerned about at this time. Is there a particular reason why you asked that question?
Yep, what's you take? I think it needs to be watched even though little expected for the time being.
Will be checking in and out...my school assignment due by midnight so it's all hands on the keyboard!
Wow. SAL has really cleared out of mid-atl!
Link
is that what he this said???
LOL! Nice one. Lurking on here sometimes does result in a very wet desk...
here is the satellite image...
That's a storm headed towards PR, ain't it, FM?
That is likely a tropical wave heading for PR. I agree with the NAM with the track, but not with the intensity. It still has to cross an area of drier air.
ahhh
thats what this blog is about....discussing waves and possible development and sometimes freaking out...and no one is yet
hello?
its getinger stronger and stronger
Link me to that site
Great image
It benefited from the wave at 44-49W which help disperse the dust alot ahead of the wave.
Could use a fleet of 100 or so barges out there at 31W
Maybe....but according to Joe Bastardi it will dissipate by winter.
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