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99L south of Jamaica close to tropical depression status

By: Dr. Jeff Masters, 2:33 PM GMT on October 22, 2012

A tropical disturbance centered about 300 hundred miles south of Jamaica (Invest 99L) is close to tropical depression status. The disturbance is nearly stationary over very warm waters of 29.5°C, and is in a moist environment. 99L has a large area of heavy thunderstorms that have a good degree of spin. These thunderstorms are steadily organizing into curved spiral bands, as seen on visible satellite loops. There are no obvious signs of a surface circulation, but an Air Force hurricane hunter aircraft will investigate 99L this afternoon to see if a tropical depression has formed.

Figure 1. Morning satellite image of Invest 99L.

Forecast for 99L
Wind shear is a low 5 - 10 knots, and is forecast to be in the low to moderate range, 5 - 20 knots, through Tuesday night. This should allow for some steady development of 99L, and there is strong model support for 99L becoming a tropical depression by Wednesday. Steering currents have collapsed, and 99L should move little today. On Tuesday, a trough of low pressure to the north of 99L is expected to turn the storm to the north or north-northeast, which should put the storm in the vicinity of Jamaica on Wednesday and Eastern Cuba on Thursday. By Friday, 99L should be in the Central or Eastern Bahamas. It is unclear at this point whether or not the trough pulling 99L to the north will be strong enough to pull the storm all the way out to sea to the northeast; a very complicated steering environment will develop late this week, and it is possible that a narrow ridge of high pressure could build in over 99L and force the storm to the west-northwest, with a potential threat to the Northwestern Bahamas and U.S. East Coast by Saturday. In their 8 am EDT tropical weather outlook, NHC gave 99L a 90% chance of developing into a tropical cyclone by Wednesday morning. 99L will be capable of bringing heavy rains of 5 - 10 inches, with isolated amounts of up to 15 inches in mountainous areas, to Jamaica and Haiti, Monday night through Thursday. Heavy rains will begin on Tuesday in Eastern Cuba, and spread northwards into the Central and Eastern Bahamas by Wednesday. Heavy rains of 3 - 6 inches can be expected in the Cayman Islands and Dominican Republic, Tuesday through Thursday.

Invest 90L in the middle Atlantic
A small low pressure system (Invest 90L) about 700 miles east-northeast of the northernmost Lesser Antilles Islands is headed northward at about 10 mph. The disturbance has a modest area of heavy thunderstorms, as seen on visible satellite images, and is struggling with cool, dry air from the upper-level low pressure system that it is trying to form underneath. This upper-level low has provided 90L the spin it needs to become a tropical cyclone, though. Wind shear is a moderate 10 - 20 knots, and is forecast to remain in the moderate range until Wednesday morning. This may allow 90L to develop into a tropical cyclone before it encounters high wind shear of 20 - 30 knots on Wednesday. It's unlikely that 90L will affect any land areas. In their 8 am EDT tropical weather outlook, NHC gave 90L a 50% chance of developing into a tropical cyclone by Wednesday morning.

The next name on the list of Atlantic tropical storms for 2012 is Sandy.

Jeff Masters


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