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97L more organized, could be a tropical depression by Wednesday

By: Dr. Jeff Masters, 12:55 PM GMT on October 05, 2010

A large region of disturbed weather (Invest 97L) is bringing heavy rain showers and strong wind gusts to the Lesser Antilles Islands and waters near Puerto Rico, the Virgin Islands, and the Dominican Republic. At 8:30am AST this morning, Charlotte Amalie in the Virgin Islands recorded a wind gust of 52 mph in a heavy rain squall. Long range radar out of Puerto Rico shows that the echoes associated with 97L are slowly growing more organized, and now have a broad circulation, centered just north of the Virgin Islands. This broad circulation is also suggested by recent visible satellite loops. Satellite imagery also shows that low-level spiral bands are slowly developing over the northern Lesser Antilles islands, to the east of the center of circulation. Martinique radar shows a modest amount of heavy thunderstorm activity over the Lesser Antilles, and rainfall amounts of 1 - 3 inches have been common in the islands over the past two days. An upper level trough of low pressure is contributing to the heavy rain showers by making the atmosphere more unstable. This same trough is also bringing moderate wind shear of 10 - 15 knots and pumping dry air into the west side of 97L. These impacts should keep 97L from developing into a tropical depression today.

Figure 1. Morning satellite image of Invest 97L.

Forecast for 97L
Tonight and Wednesday morning, the upper-level trough to 97L's west is forecast to weaken. This will significantly reduce the amount of dry and and wind shear affecting the storm, and 97L should be able to organize into a tropical depression by Wednesday afternoon, when the Hurricane Hunters are scheduled to pay their first visit. The SHIPS model predicts that wind shear over 97L will fall to the low range, 5 - 10 knots, by Wednesday. The current northwest motion of 97L should keeps its center from crossing Hispaniola. All of the major computer models show development of 97L into a tropical depression by Wednesday night, just north or northeast of Hispaniola. By Thursday, a passing trough of low pressure is expected to pull 97L to the northeast away from Hispaniola, and rains should end by Friday on the island. NHC is giving 97L a 60% chance of developing into a tropical depression by Thursday. Since the center of this potential Tropical Storm Otto is not expected to pass over any land areas, rainfall will be the primary threat. The Dominican Republic and Puerto Rico are most at risk of flooding rains from the storm, which could bring 3 - 6 inches of rain through Thursday. Haiti and the Turks and Caicos Islands are at less risk, since the dry air and lingering wind shear from the upper-level trough to 97L's west will keep heavy thunderstorm development lower on the west side of the storm. Flash flood watches are posted for the Virgin Islands and Puerto Rico for today through Wednesday, and heavy rains will continue in the northern Lesser Antilles Islands through Wednesday.

Elsewhere in the tropics
Most of the models indicate the possibility that a strong tropical disturbance or tropical depression forming in the Southern Caribbean, off the coast of Nicaragua, 6 - 7 days from now.

"Hurricane Haven" airing this afternoon
My live Internet radio show, "Hurricane Haven", will be airing again today at 4pm EDT. The call in number is 415-983-2634, or you can post a question to broadcast@wunderground.com. Be sure to include "Hurricane Haven question" in the subject line.

Today's show will be about 20 minutes, and you can tune in at http://www.wunderground.com/wxradio/wubroadcast.h tml. The show will be recorded and stored as a podcast. I'll discuss 97L, and the odds of the U.S. getting a major hurricane strike during what remains of hurricane season.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.