93L Fighting Dry Air, But Could be a Tropical Depression

By: Dr. Jeff Masters , 1:47PM,GMT on July 31,2014

An area of disturbed weather located near 11°N, 52°W at 8 am EDT Thursday, about 650 miles east of the Lesser Antilles Islands (93L), has maintained a well-organized surface circulation and has now developed enough heavy thunderstorms to potentially be classified as a tropical depression later today. Visible satellite loops on Thursday morning showed that the surface circulation of 93L was exposed to view, with a modest clump of heavy thunderstorms on the south side. The storm was fighting moderate wind shear of about 15 knots, due to strong upper-level winds out of the north. These winds were driving dry air to the north of 93L into the circulation, keeping heavy thunderstorms from forming on the north side. An ASCAT pass at 9:11 pm EDT Wednesday night showed top surface winds near 35 mph, just below tropical storm-force. Water vapor satellite loops and the Saharan Air Layer analysis showed that a large amount of dry air lay to the north and west of 93L. Ocean temperatures were about 28°C, which is 2°C warmer than the typical 26°C threshold for development. An Air Force C-130 hurricane hunter aircraft will investigate 93L on Thursday afternoon, and the NOAA Gulfstream IV jet is scheduled to fly a dropsonde mission on Thursday afternoon around the storm.


Figure 1. Latest satellite image of 93L.

Forecast for 93L
Moderate wind shear of 10 - 20 knots is expected to affect 93L for the next five days, according to the 12 UTC Thursday forecast from the SHIPS model. With the atmosphere around 93L now quite dry, the storm will have to work hard to insulate itself from disruptive dry air incursions. Sea Surface Temperatures (SSTs) will stay roughly constant at 28°C. The models agree that 93L 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. Our three reliable models for predicting tropical cyclone genesis, the GFS, European, and UKMET models, predicted in their 00Z Thursday runs that 93L would be no stronger than a 40 mph tropical storm as it passed through the islands, and then dissipate early next week near the Southeast Bahama Islands. The GFDL and HWRF models predicted in their 06Z Thursday runs that 93L would be a weak tropical storm as it passed through the islands, intensify some after passing over Puerto Rico on Saturday night, but turn to the north, missing the Bahamas. In their 8 am EDT Thursday Tropical Weather Outlook, NHC put the 2-day and 5-day odds of development at 70%. The Thursday morning runs of our top four models for predicting intensity, the LGEM, DSHIPS, GFDL, and HWRF models, predicted that 93L would have top sustained winds between 40 - 55 mph on Friday - Saturday as it affects the Lesser Antilles Islands, Virgin Islands, and Puerto Rico. I predict that 93L will continue to struggle with dry air as it passes through the islands on Friday and Saturday, with top sustained winds between 35 - 50 mph.

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 Thursday 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.

Jeff Masters