Monthly Archives: July 2012

Invest 99L forms. Will it become Ernesto?

After a month of tranquility across the Atlantic, the Cape Verde season has kicked into gear across the Mean Development Region. On one end, a tropical wave is bringing showers and storms to the Lesser Antilles and to Puerto Rico. On the other, convection persists along the monsoonal trough, which essentially is an elongated area of low pressure that circles the globe between 5°S and 15°N. The monsoonal trough is currently located around 10°N in the Atlantic and will continue to envelop the Cape Verde season during the next 2-3 months.  In the middle is an area of low pressure embedded in a tropical wave in the central Atlantic.

99L currently has winds of 25 mph and a pressure 1009 millibars. 99L is rather disorganized with its ‘center’ embedded in the deepest thunderstorms, but the future is bright. In the short term, 99L will move just north of due west around the southern periphery of the Bermuda High. A quick look at shear analysis, water vapor forecasts and upper level flow shows quite favorable conditions at least in the short term, and very warm waters will supplement a favorable forecast for slow growth. Below is the current satellite image of 99L (11:20p ET) with the current NHC position overlayed (00z/8:00p ET). The deep oranges and reds are tall/strong thunderstorms and the cooler colors to the north represent clear skies and the Saharan Air Layer, which should remain to the north with no inhibitory effects.

Most models continue to be favorable with 99L with slow nightly development. I expect the next couple of days will be down days and up nights that will heavily lean toward diurnal thunderstorm development and organization. I am forecasting tropical depression development on either late Wednesday or on Thursday with the possibility of tropical storm strength by Friday if favorable conditions persist. My only hesitation by days 5 and 6 are an uptick in upper level shear and any land impact. 99L will be moving into the Caribbean on Friday or Saturday, likely in the central Lesser Antilles due to its current southern position. The system will continue to be guided westward by the Bermuda High for the next 4-6 days before turning WNW ahead of a trough that will be in place across the eastern United States.

As far as intensity goes, it is too early to forecast, but with favorable conditions in play slow development every day is likely. Upon reaching the lesser Antilles, I believe that a low-balled forecast of 40-45 mph is reasonable, but for those living in the islands be prepared for a higher forecast. Heavy rain and high seas can be expected whether or not this gets upgraded to a tropical cyclone.

Also of note for Floridian weather, that tropical wave I mentioned on the western side of the Atlantic…expect that to be seen by Friday in the form of enhanced thunderstorm chances and slightly lower high temperatures.

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