Puerto Rico braces for a December surprise: 94L

The hurricane season of 2007 over may not be quite over. An area of disturbed weather about 200 miles east of Puerto Rico, designated Invest 94 by NHC, may develop into a subtropical or tropical storm. The system developed a surface circulation near 18N 63W this morning, as seen on visible satellite loops. The heavy thunderstorm activity is displaced 100-300 miles to the north of the center, making 94L a subtropical system. Long range radar out of Puerto Rico shows some disorganized bands of rain affecting the Virgin Islands, Puerto Rico, and surrounding waters, but these bands are not well-organized. Water temperatures in the region are about 26-27° C, which is barely warm enough to support formation of a tropical storm. Last evening's 8:12 pm EST QuikSCAT pass showed top winds of 30-35 mph. Unfortunately, this morning's QuikSCAT and ASCAT passes both missed 94L. Wind shear is 20-25 knots over the disturbance, which is too high to allow anything but slow development. However, 94L is very close to being a subtropical depression, and only a slight increase in organization would be needed to call it a subtropical depression.


Figure 1. Latest satellite image of Invest 94.

Here's what NHC had to say about the disturbance:

Special tropical disturbance statement
1100 am EST Mon Dec 10 2007

Satellite images and surface reports indicate that a closed surface circulation has developed in association with the broad area of low pressure now centered about 200 miles east of Puerto Rico. Shower activity with the low remains disorganized...however...with the strongest thunderstorms located a couple hundred miles north and northeast of the center. While a tropical or subtropical cyclone could still form during the next 24 hours...upper-level winds are expected to become gradually less favorable for development over the next couple of days. Regardless of whether the low develops further...it could produce heavy squalls and gusty winds of near gale force across the Virgin Islands and Puerto Rico later today and tonight as it moves westward or west-southwestward at about 20 mph. Heavy rains over Puerto Rico and Hispaniola could cause life-threatening flash floods and mud slides...and interests in these areas should continue to monitor the progress of this system.

Wind shear is expected to remain in the 20-25 knot range Monday, which may be low enough to allow 94L to organize into a subtropical depression or subtropical storm today. Regardless, 94L will affect Puerto Rico today and tonight similar to how a tropical depression would, with sustained winds of 30 mph gusting to 45 mph, and heavy rains up to 5 inches. On Tuesday, these conditions will spread to the Dominican Republic and Haiti. The high terrain of Hispaniola should disrupt 94L, and wind shear is also expected to increase. By Wednesday, 94L will probably bring heavy rain to Jamaica. The GFDL model indicates that wind shear will drop and 94L will organize into a tropical storm on Wednesday. The model forecasts that the storm will intensify and move west-southwest to threaten the northern coast of Honduras on Friday. None of the other models develop 94L, though. Wind shear is expected to be low enough to support tropical storm formation in the Western Caribbean on Wednesday-Thursday, if there is anything left of 94L after its encounter Tuesday with Hispaniola. The Hurricane Hunters are not on call to fly 94L at all, and I don't expect this storm will survive intact enough to become a tropical storm in the Western Caribbean later this week.

I'll have a update later today.

Jeff Masters