From Dr. Jeff Masters, Weather Underground

 A tropical wave embedded in a large trough of low pressure (Invest 99L) covers a large portion of the Central Caribbean between Hispaniola and the northern coast of South America.

This storm has the potential to be a dangerous rainfall threat for Haiti, Jamaica, and eastern Cuba. The disturbance is headed west at less than 5 mph, is over very warm waters of 29°C, and is in a moist environment.

99L has a large area of heavy thunderstorms that are beginning to show some organization, as seen on visible satellite loops. Wind shear is a low 5 – 10 knots, and is forecast to be in the low to moderate range, 5 – 20 knots, through Tuesday. This should allow for some steady development of 99L, and there has been a good deal of model support for 99L becoming a tropical depression by Wednesday.

Steering currents favor a continued slow westward movement for 99L through Tuesday, which will bring a multi-day period of heavy rains to Haiti, Jamaica, and eastern Cuba beginning on Sunday.

The Dominican Republic may also see some heavy rains, though not as great.

On Wednesday, a strong trough of low pressure will approaching the Eastern U.S., and the GFS and ECMWF models predict that this trough will turn 99L to the north, bringing the storm into the southeast Bahama Islands on Thursday or Friday.

In their 8 am EDT tropical weather outlook, NHC gave 99L a 30% chance of developing into a tropical cyclone by Monday morning. I put these odds higher, at 40%.**

An Air Force hurricane hunter aircraft is scheduled to investigate 99L on Sunday afternoon.

For more information on this story go to:

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2265

**They now say 70% at 8.00 AM EDT

TROPICAL WEATHER OUTLOOK

NWS NATIONAL HURRICANE CENTER MIAMI FL

8.00 am EDT Sun Oct 21 2012

For the North Atlantic…Caribbean Sea and the Gulf of Mexico…

1. Showers and thunderstorms have been increasing over the past few

hours near a tropical wave and broad area of low pressure over the central Caribbean Sea.

Environmental conditions appear conducive for a tropical depression to form during the next couple of days… and this system has a high chance…70 percent…of becoming a tropical cyclone during the next 48 hours as it moves slowly westward.

Heavy rains from this disturbance are likely to spread over Jamaica…eastern Cuba…and Hispaniola over the next several days.  These rains could produce life-threatening flash floods and mud slides…especially in areas of high terrain.