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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ok.. what if your avatar is already loaded?? Then what? Thank you again in advance...:)
A west track toward PR and the Antilles is very probable.
If you load another one it will replace the existing one.
I think it takes time for it to offically change over once you make the change but dont quote me on that.
26-27 degrees is a good temp to maintain a storm, albeit a weak one, but is not very conducive for a developing one. And yes, I know there are a very few exceptions. The NHC's use of "any development will be slow to occur" is very apt right now bc in 26-27 degree water that is the only kind of development that can occur.
You are correct..Admin. has to approve.
For instance, if you take the factors:
1) Shear
2) Initiating Disturbance quality
3) Low to Mid level moisture
4) Distance from Equator
5) Water Temp
6) Temp gradient with altitude
7) other?
Then, for each factor you have a scale... say from 1 to 3. For instance:
Shear:
1 - 0 to 15kts
2 - 15 to 30kts
3 - 30+ kts
Then, weight each of the factors based on importance to formation.
The idea being if you sum all the weighted factors (factor times the weight), the maximum score that can be achieved is 100%.
I think if something like this existed, it would make it a lot easier for many of the folks (lurking or not) to understand the potential for any development of a tropical system.
I dont totally disagree with you. I guess maybe I should have said it differently. 26C is the min and you are right, slow to little development. However, the current 26-27C will help maintain the wave until it hits the warmer temps. I am not expecting a hurricane this far away but if it maintains its form it has a small shot. The shear is really what is going to destroy it if the ridge does not build in. If the ridge builds in, shear is reduced, it will survive to hit those 28-29C temps and start to grow into a TC.
Do you think the CMC's run could come true.
Thank you..:)
*
could you send me the cmc model run
Tropical Cyclone Formation Probability Guidance Product
dont fot get
LOL!!!!
Forecasting a tropical cyclone's magnitude is complicated. However once it gets to a more moist or an extremely moist environment, it should be a able to disperse its energy with greater efficacy, and that may help increase the size. Interaction with other cloud masses or a frontal system tends to increase a tropical cyclone's size as a result of increasing convection caused by upper level diffluence. Wilma became enormous after interacting with the fronts that advected toward FL. Ike gained its magnitude probably because of the fact that the center remained overland in Cuba, but the surroundings remained over water. This leads to increasing cloud cover in the surroundings than at the center itself. Once over water, Ike hard time coalescing itself, because of this. The primitive satellite images of Ike once it exited Cuba showed a quasi-pinhole eye feature. This indicates validates that the primary core was small, and really difficulty fusing all of the surrounding energy. Eventually, the overall storm broadened, and finally began to efficaciously adapt to its new size. Nevertheless it was too late, because it was already close to landfall. Notice how Ike looked much better before landfall in Texas.
LOL I got carried away. Sorry WS
Thanks... do these correlate at all to the percentage given by the NHC?
That's some heavy dust in the Caribbean
Not the TCFP. NHC is the forecaster's analysis.
Meteosat Long-Wave Difference (SAL)
(CIRA Personnel Only)
So were wave watching?
Had some real nasty storms roll through here this evening. Which lead to this interesting feature moving off the Carolinas.
¿~)
Going to my hurricane kit and getting the head on..apply directly to the forehead.
that wave is now at 50w
Evening SJ
Almost looks like a waterspout formed offshore in that feature. Thought that was you under Portlight, don't think Press could post a link. LOL
how many miles
is it away from the Islands?
almost 1500
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