Disturbances 92L, 93L a threat to the Lesser Antilles Islands

By: Dr. Jeff Masters , 2:35 PM GMT on August 11, 2008

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A tropical wave near 12N 49W, about 800 miles east of the Lesser Antilles Islands (92L), has changed little since Sunday. A QuikSCAT pass at 5:12 pm EDT last night revealed a large, elongated circulation, with top winds of 30 mph. This morning's QuikSCAT pass missed 92L. Wind shear is a low 5-10 knots over the disturbance. Satellite loops show a limited amount of heavy thunderstorm activity that has decreased slightly since Sunday. Water vapor satellite loops show that 92L is at the edge of a large area of dry air and Saharan dust to its west and north, and this dry air has been drawn into the center of circulation, disrupting the storm.


Figure 1. Latest satellite image of 92L.

The forecast for 92L
Water temperatures are a warm 28.4°C and forecast to increase to 28.8°C three days from now. Wind shear is forecast to remain below 10 knots for the next five days. The environment appears favorable for intensification, except for the dry air to the west. Most of the models are indicating 92L will have trouble developing. The GFDL model is the most aggressive, intensifying 92L to a 55 mph tropical storm that passes just north of the northernmost Lesser Antilles Islands on Thursday. In contrast, the HWRF model does not develop 92L at all. The National Hurricane Center is giving 92L a medium (20-50% chance) of becoming a tropical depression by Wednesday morning. This is a step down from their forecast last night of a high (>50%) chance of developing, and reflects the uncertainty that 92L will be able to get organized in the face of significant dry air to its west. Nevertheless, residents of and visitors to the northern Lesser Antilles should anticipate the possibility of a tropical depression or minimal tropical storm arriving in the islands as early as Wednesday (though Thursday is more likely). The southern islands are less likely to be affected.

Disturbance 93L off the coast of Africa
A tropical wave near 11N 29W (93L), about 300 miles west-southwest of the Cape Verde Islands, has developed a modest amount of heavy thunderstorm activity overnight. A QuikSCAT pass at 2:34 am EDT this morning showed a closed circulation, with top winds of 30 mph. Wind shear is a high 20 knots over the disturbance. Water vapor satellite imagery shows that 93L is embedded in a large area of moist air, with some dry air and Saharan dust to its north.


Figure 1. Latest satellite image of 93L.

The forecast for 93L
Water temperatures are a warm 28° and forecast remain near 28°C for the next five days. Wind shear is forecast to drop below 10 knots by Tuesday night, and remain below 10 knots through Friday. Dry air may begin to be a problem for 93L beginning on Wednesday, as it works its way a bit further to the north where a dry Saharan Air Layer (SAL) exists. Odds are, 93L will be able to develop into a tropical depression by Friday. Most of the computer models are developing 93L into a tropical storm by early next week, and put it on a track to skirt the northernmost Lesser Antilles Islands. The National Hurricane Center is giving 93L a medium (20-50% chance) that it will be a tropical depression by Wednesday morning. Residents of and visitors to the Lesser Antilles should watch this system, as it has the potential to bring tropical storm conditions to the islands early next week.

Elsewhere in the tropics
Several of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 7 days from now.

This afternoon, I'll post an analysis of the expected wind shear and dry air over the tropical Atlantic during the coming week.

Jeff Masters

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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