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By Dr. Jeff Masters From Weather Underground

An area of disturbed weather located near 9°N, 45°W at 8 am EDT Wednesday, about 1150 miles east of the Lesser Antilles Islands (93L), has the potential to develop into a tropical depression by Thursday, but is struggling with high wind shear today. Visible satellite loops on Wednesday morning showed 93L had a well-defined surface circulation and some low-level spiral bands. However, infrared satellite images showed heavy thunderstorm activity was very limited, and the storm is fighting high wind shear of about 20 knots. Water vapor satellite loops and the Saharan Air Layer analysis showed that while there was some dry air in the vicinity of 93L, the bulk of the dry Saharan Air Layer lay to the north. Ocean temperatures had warmed since Tuesday,

and were about 28°C, which is 2°C warmer than the typical 26°C threshold for development.

Figure 1. Latest satellite image of 93L.

Figure 2. Analysis of the Saharan Air Layer (SAL) made at 8 am EDT Wednesday July 30, 2014 using data from the Meteosat-9 satellite. Dry, dusty air was present from the coast of Africa westwards across the tropical Atlantic, but was fairly well separated from tropical disturbance 93L. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Forecast for 93L

The high levels of wind shear affecting 93L on Wednesday morning were expected to diminish to a moderate 10 – 15 knots by Wednesday night, according to the 12 UTC Wednesday forecast from the SHIPS model. With the atmosphere around 93L reasonably moist, this may allow the system to become a tropical depression as early as the 11 pm EDT Wednesday NHC advisory. Thursday morning is probably a more likely time for classification as a TD, though. Sea Surface Temperatures (SSTs) will stay roughly constant at 28°C. Two of our three reliable models for predicting tropical cyclone genesis, the GFS and UKMET models, predicted in their 00Z Wednesday runs that the disturbance would develop into a tropical depression by Friday, but were much less aggressive about strengthening the storm than in previous runs. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC put the 2-day and 5-day odds of development at 70% .

All of the models predict that the disturbance will continue west-northwest at 13 – 19 mph for the next four days. The UKMET and the European models offer the fastest solution, predicting that the disturbance will arrive in the northeast Lesser Antilles Islands on Friday evening, move over Puerto Rico on Saturday evening, and approach the Southeast Bahamas and Turks and Caicos Islands on Sunday evening. The GFS is slower and more to the northeast, predicting a Saturday morning arrival in the northern Lesser Antilles, with passage about 100 miles northeast of Puerto Rico occurring on Sunday morning. Dry air to the north of 93L will likely interfere with development throughout the week, and the atmosphere surrounding 93L will grow drier as the storm progresses west-northwest. The moderate levels of wind shear forecast to occur will be capable of driving this dry air into the core of the system, disrupting formation. The Wednesday morning runs of our two top statistical models for predicting intensity, the LGEM and DSHIPS models, forecast that once 93L became a tropical depression, it would intensify into a hurricane within 3.5 days. However, the dynamical GFDL and HWRF models, which made good intensity forecasts for Hurricane Arthur, were much less bullish. The Wednesday morning runs of these models predicted that 93L would never reach hurricane strength. I give a 10% chance that 93L will be a hurricane on Saturday when it makes its closest approach to the Lesser Antilles Islands. If 93L hits the Northern Lesser Antilles as a wet Tropical Storm Bertha (not a hurricane), the storm could be more boon than bane for the islands. The Northeast Caribbean suffered its driest June in recorded history last month, according to NOAA, and many of the islands have significant drought problems.

The GFS and European models continue to agree on the long-range fate of 93L. The great majority of the 20 members of the 00Z Wednesday runs of the European and GFS ensemble models (which run at low resolution 20 times with slightly different initial conditions to show a range of possible outcomes) showed 93L taking a northwesterly track early next week in response to a strong trough of low pressure over the Eastern United States, then recurving to the north without hitting the mainland U.S. coast.

The Hurricane Hunters to study 93L

Originally, a fleet of five hurricane hunter aircraft were headed to the Caribbean today to intercept 93L, according to the NOAA/HRD blog and the NHC recon Plan of the Day. Two Air Force C-130s are still headed to the islands, and will begin flying alternating missions into 93L on Thursday afternoon. The three NOAA aircraft that were scheduled to deploy today have now had their missions cancelled.

For more on this story go to: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2741

Other images from National Hurricane Center

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