Tropical depression likely by Tuesday in Western Caribbean

A vigorous surface circulation has developed in association with a broad area of low pressure over the Western Caribbean (94L). A pass from the European ASCAT satellite at 11:02 am EDT showed that the surface circulation had gotten much less elongated, compared to this morning's QuikSCAT pass. Top winds were only about 10 mph in the 11 am ASCAT pass, but have no doubt increased since then. Satellite loops show a very large surface circulation covering the entire Western Caribbean, with a concentrated area of thunderstorms forming near the center of circulation. Surface pressures remain very low over the entire Western Caribbean. Wind shear is less than 10 knots, and is expected to remain less than 10 knots through Thursday. It is likely that this system will form into a tropical depression on Tuesday, despite the very large amount of atmosphere it is trying to spin up.

Steering currents are weak in the Western Caribbean, but most of the models show a slow motion to the west or northwest that will take 94L over Belize or Mexico's Yucatan Peninsula by Thursday. It is unlikely 94L will have time to become a hurricane before it moves over the Yucatan, and the main threat from the system will be heavy rain. These rains may cause significant flooding problems in Belize and Mexico's Yucatan. Heavy rains may also affect northeast Guatemala, but should not cause significant flooding.

There is a strong trough of low pressure forecast to swing across the U.S. this week, which could turn 94L northwards into Western Cuba, the Florida Keys, or Southwest Florida. However, this is unlikely, since none of the reliable computer models are forecasting such a turn.

It is likely that 94L will eventually emerge into the Gulf of Mexico or Western Caribbean after spending a few days over the Yucatan. When it does so, it will probably be intact enough to re-strengthen, since it is such a large system. Its long-term fate it highly uncertain, as the steering currents are weak and the storm's intensity will be controlled by interaction with land.

Elsewhere in the tropics
A nearly stationary tropical disturbance a few hundred miles north of Puerto Rico is generating a large area of thunderstorms. The region is under 15-20 knots of wind shear, due to strong upper level winds from the west. These winds are blowing the heavy thunderstorm activity downwind, to the east of the elongated surface circulation apparent on visible satellite imagery. Wind shear is expected to remain 15-30 knots over the region over the next three days. The high shear should discourage any significant development.

I'll have an update in the morning.

Jeff Masters