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September hurricane outlook; Bill Proenza returns to his old job

By: Dr. Jeff Masters, 1:17 PM GMT on September 11, 2007

A strong tropical wave near 11N 43W, halfway between Africa and the Lesser Antilles Islands, is headed west-northwest at 10-15 mph. This system (91L) has the potential to develop into a tropical depression later this week. This morning's QuikSCAT pass continued to showed a broad, elongated circulation and top winds of 25 mph. Satellite loops show no improvement in organization has occurred since yesterday--91L consists of some disorganized clumps of heavy thunderstorm activity. The disturbance is under about 10-15 knots of wind shear. Shear is forecast to remain near 15 knots today and Wednesday, then drop below 5 knots on Thursday. I expect this will allow 91L to develop into a tropical depression on Thursday. The HWRF brings it to a Category 3 hurricane by Sunday, at a position near 19N 58W, about 500 miles east-northeast of Puerto Rico. This is too aggressive an intensification rate, but I expect 91L will be at least a strong tropical storm by Sunday. The 06Z run of the GFDL model is more believable, making 91L a 55 mph tropical storm about 800 miles east of Puerto Rico on Sunday. This storm is definitely a threat to the Lesser Antilles Islands. It is too early to say if the northern islands are more at risk, as the current model runs are indicating. The system may represent a threat to the U.S. East Coast ten or more days from now, but there is no way to judge the likelihood of this.

Gulf of Mexico
The area of disturbed weather in the southern Gulf of Mexico has diminished overnight, but does have the potential to grow again today before moving ashore over Texas and northern Mexico tonight and Wednesday morning, bringing heavy rain. New thunderstorm activity building off the Gulf Coast of Florida is very disorganized and is not a threat to develop.

Rest of September outlook
We're halfway. September 10 marks the peak of hurricane season in the Atlantic, and the frequency of hurricanes and tropical storms starts to decline now as water temperatures begin to cool and wind shear begins to rise. However, more activity typically occurs after September 10 than before, due to the fact that the ocean takes a long time to cool off. Given that we've already had 7 named storms so far this year, another 7 or 8 named storms are probably in order. The latest Dr. Bill Gray/Phil Klotzbach monthly forecast calls for 8 more named storms, with 5 of them hurricanes--two of these being intense hurricanes. They anticipate a later than usual end to hurricane season, due to the cooler than average conditions in the Eastern Pacific that might signal the beginning of a La Ni�a event. La Ni�a events typically bring lower than average wind shear to the Atlantic. Since high wind shear is usually what brings an end to hurricane season (this was certainly the case last year), development of a La Ni�a event over the next few months should act to prolong this year's hurricane season.

Figure 1. Average Atlantic hurricane activity.

Sea Surface Temperatures (Figure 2) are near average to 0.5 �C above average over much of the Main Development Region (MDR) for hurricanes (10� N to 20� N, from Africa to Central America). While this is nothing like the record SSTs observed in 2005, this is still a lot of extra heat energy. SSTs remain depressed over the southern Gulf of Mexico and western Caribbean, due to the cold water wake of Hurricane Dean. Wind shear the past 11 days (Figure 3) has been below normal over most of the MDR. These conditions are expected to continue over at least the next two weeks, according to the latest forecast from the GFS model. African dust activity has been quite low the past month, and I don't see any changes to the general circulation pattern that would change this. Steering current patterns are expected to remain the same as we've seen since since late July, with a series of weak troughs and ridges rippling across the Atlantic, and no major troughs or ridges locking into place. This steering pattern favors a near-normal chance of hurricane strikes for the entire Atlantic. Due to the weak nature of the troughs of low pressure expected, we'll have fewer recurving storms that miss land than normal. Indeed, all but one of the seven named storms we've had this year have affected land (Chantal was the exception).

Figure 2. Sea Surface Temperature (SST) departure from average for September 10, 2007. Image credit: NOAA.

Figure 3. Departure of wind shear from average for the past 11 days (in meters per second). Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). Note that wind shear has been below average over most of the tropical Atlantic the past 11 days. Image credit: NOAA/CPC.

Bill Proenza returns to his old job
Bill Proenza will return as head of the Southern Region of the National Weather Service, according to an article published in the a href=http://www.miamiherald.com/news/hurricane/sto ry/233190.html yesterday. Mr. Proenza was director of the National Hurricane Center between January and July this year, until he was forced out by a dispute over his claims about the rationale for replacing the aging QuikSCAT satellite and an unprecedented revolt by his staff. He will return to the job he had before the NHC directorship, managing nearly 1,000 National Weather Service employees in New Mexico, Texas, Oklahoma, Arkansas, Louisiana, Mississippi, Tennessee, Alabama, Georgia, Florida, the Commonwealth of Puerto Rico and the U.S. Virgin Islands. Proenza resumes his duties on September 23. Proenza's return to his old job was pushed by two powerful congressmen, Democratic Reps. Brad Miller of North Carolina and Nick Lampson of Texas.

I'll have an update Wednesday morning.
Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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