Lesser Antilles wave a threat to become a tropical depression

A tropical wave moving westward at 15-20 mph near 13N 57W, about 300 miles east of the Lesser Antilles Islands, has made a significant comeback this morning after losing its spin and most of its heavy thunderstorm activity last night. The system has re-gained its spin, and a new blow-up of intense thunderstorms has developed over the circulation center. It remains to be seen if the storm can hang onto this thunderstorm activity; 10-20 knots of wind shear are still interfering, and this system has had a history of alternately looking organized, then disorganized, as the shear waxes and wanes. Strong upper-level westerly winds from an upper-level low pressure system to the north are responsible for this shear, and this low is expected to continue to bring significant amounts of shear over the system for the next two days. It is possible that the shear may weaken enough to allow a tropical depression to form in the next day or two, but it will be a struggle for this system to get organized. Pressures are falling at the two buoys about 100 miles to the north of the storm's center, though, and peak winds at 9am EDT at one of these buoys was a sustained 28 mph. We don't have a recent QuikSCAT satellite pass to judge the winds that way, unfortunately. There are the beginnings of some upper level outflow apparent on satellite imagery, but no real low-level spiral banding occurring yet.

The wind shear is greatest to the system's north, so the further south it stays, the more likely it is to develop. The Saharan Air Layer (SAL) of very dry air is 100-200 miles to the system's north, which is probably far enough away that it will not impact it over the next day or two.

Last night's GFDL model predicted that the wave will develop by Thursday into a weak tropical storm, which will move through the Caribbean Sea to a point south of Haiti on Saturday, where high wind shear will dissipate it. None of the other computer models develop the storm at all.

The wave should move through the Lesser Antilles Islands tonight and Thursday morning, bringing winds of 30-40 mph and heavy rain to Barbados, St. Lucia, Martinique, and surrounding islands. Puerto Rico may get socked on Friday with these conditions, and the Dominican Republic on Saturday. The Hurricane Hunters are on call to investigate the wave today at 2pm EDT today, and I'll have an update this afternoon on what they find.


Figure 1. Preliminary model tracks for the mid-Atlantic tropical wave.

Elsewhere in the tropics
Clouds and showers stretching from the Bahamas through South Florida are associated with an upper level low pressure system. High wind shear, dry air, and cool air temperatures are expected to keep this area from developing. A broad non-tropical low pressure system located about 800 miles southwest of the Azores is drifting southward, and is not expected to develop over the next two days. Some of the computer models are forecasting that development is possible by Sunday or Monday, though.

Jeff Masters