Lesser Antilles disturbance fizzles, but may still cause trouble

By: Dr. Jeff Masters , 2:26 PM GMT on August 13, 2008

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A tropical wave about 300 miles east of the Lesser Antilles Islands (92L) fell apart last night, but is beginning to increase in organization today. We don't have a QuikSCAT pass this morning to look at, but visible satellite loops show some rotation at low levels (though not a full closed circulation), and one clump of heavy thunderstorms near the center of rotation. The storm has no low-level spiral bands or upper-level outflow, and is in a very fragile state. Moderate wind shear of 15-20 knots or a major injection of dry air could doom the system. Right now, it doesn't appear that wind shear will be high enough to destroy 92L--shear is only 5 knots, and expected to remain less than 10 knots for the next 5 days. Water vapor satellite loops show that a large area of dry air and Saharan dust surrounds 92L on all sides, and this dry air was responsible for 92L's severe degradation last night and early this morning. Dry air remains 92L's greatest enemy.

The forecast for 92L
Watching the the model forecasts for 92L over the past three days has, for me, been akin to watching the latest Batman movie, The Dark Knight. As the Joker prepares for one of his deadly pranks, the music rises in pitch and volume, and the audience nervously waits to see what terrible mayhem the Joker has planned next. Like music in the movie, the reliable GFDL model forecasts of 92L the past three days have risen in pitch and volume. The GFLD has been forecasting successively stronger hurricanes each day, culminating in yesterday afternoon's run predicting a Category 3 hurricane plowing through the Bahama Islands towards Florida this weekend. Well, our Batman--dry air--has come to the rescue this time, significantly disrupting 92L. However, it remains to be seen if the Joker--92L--has one more trick up its sleeve. The GFDL model is still calling for 92L to develop into a borderline Category 1 hurricane by early next week, as is the latest run of the SHIPS intensity model. The other models are less gung-ho, and most of the models foresee that 92L will come close enough to the high mountains of the Dominican Republic to cause the storm trouble.

The National Hurricane Center is giving 92L a medium (20-50% chance) of becoming a tropical depression by Friday morning. A few showers from 92L have already begun to spread over the northern Lesser Antilles Islands this morning, and the bulk of the storm will spread over the islands tonight and Thursday. By Friday, 92L will be affecting Puerto Rico and the eastern Dominican Republic. Haiti, the Bahamas, and eastern Cuba can expect heavy rains Saturday or Sunday, and 92L could affect Florida early next week. The Hurricane Hunter mission for today was canceled, but a new mission is scheduled for Thursday if the storm overcomes its dry air problems.

Links to follow
Martinique radar
Wundermap for the northern Lesser Antilles Islands

Disturbance 93L off the coast of Africa
A tropical wave (93L) about 700 miles west of the Cape Verde Islands has gotten better organized since yesterday Visible satellite loops show a small clump of heavy thunderstorms near the swirling center of circulation, but no low-level spiral bands or upper-level outflow yet. A large area of stratocumulus clouds is visible to the northwest, indicating the presence of cool, dry, stable air. This stable air is inhibiting development of 93L. Wind shear has been a risen to 10-15 knots today. Water vapor satellite imagery shows dry Saharan air to the northwest.

The forecast for 93L
Water temperatures have cooled to a marginal 26.5°, but will slowly warm as 93L continues westward. Wind shear is forecast to remain below 20 knots through Friday afternoon, then increase to 30 knots. The National Hurricane Center is giving 93L a medium (20-50% chance) that it will be a tropical depression by Friday morning. I believe that this is too high, and 93L has a less than 20% chance of becoming a tropical depression this week. There is too much stable dry air to overcome between now and Friday, followed by high wind shear late in the 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 3-5 days from now.

I'll have an update this afternoon between 4pm and 5 pm EDT.
Jeff Masters

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About

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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