Western Caribbean disturbance a threat to develop

Heavy thunderstorm activity continues to flare up in association with a broad area of low pressure over the Western Caribbean (94L). This morning's QuikSCAT pass showed a 300 mile-long line of converging surface winds north of the Honduras coast (Figure 1), but not a closed circulation. Top winds were about 25 mph. Satellite loops show fewer heavy thunderstorms than yesterday, but it appears that this activity is starting to get organized. Some low-level banding of the thunderstorm activity is occurring over the Cayman Islands, to the northeast side of the center of low pressure. Surface pressures remain very low over the entire Western Caribbean, but have not fallen since Saturday. Wind shear is about 10 knots, and is expected to remain 10 knots or below through Wednesday. The low surface pressures, light wind shear, and warm ocean waters are all very favorable for formation of a tropical depression. The Hurricane Hunter flights scheduled for this afternoon and tonight were canceled, and have been rescheduled for Tuesday. I expect a tropical depression will form in the next 1-3 days, most likely on Tuesday.

Steering currents are weak in the Western Caribbean, and most of the computer models forecast that 94L will wander erratically for a week or longer in the region. A slow motion to the west or northwest is predicted for the next three days, which may bring the storm over the Yucatan Peninsula late this week. There is a strong trough of low pressure forecast to swing across the U.S. this week, which could pull 94L northwards across Western Cuba, the Florida Keys, or Southwest Florida, as forecast by the HWRF model. However, 94L would have to form quickly and grow large to "feel" the influence of this trough, and I estimate there is only a 30% chance that the trough will be able to pull 94L northwards over Florida.

Residents of the Cayman Islands, Cuba, Mexico's Yucatan Peninsula, Belize, northern Honduras, and Jamaica can expect occasional heavy rain squalls over the next 1-3 days from this storm. This activity could spread into the Florida Keys by Wednesday.


Figure 1. High-resolution (12.5 km) QuikSCAT pass from 7:34 am EDT Monday October 8, 2007. A 300 mile-long line of converging winds is apparent in association with disturbance 94L. Image credit: NOAA/NESDIS/ORA.

Elsewhere in the tropics
A tropical disturbance a few hundred miles north of Puerto Rico is generating a large area of thunderstorms. QuikSCAT data from this morning shows an elongated circulation near 25N 66W, and top winds of 25 mph. The region is under 20 knots of wind shear, and wind shear is expected to remain 20-30 knots over the region over the next three days. The high shear should discourage any significant development.

I'll have an update Tuesday morning.

Jeff Masters