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Atlantic Tropical Weather Update 9/2/09

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There has been a small but fairly significant change to Tropical Storm Erika’s forecast.  But let’s start with her vitals:

Center located at 16.5N, 60.4W; moving W @ 10mph; maximum sustained winds of 40mph; minimum central pressure of 1008mb.

Tropical Storm Erika’s center location is just offshore a few of the northeast Windward Islands: Guadeloupe and Antigua are closest to her circulation.  That positioning is well to the west of where she was yesterday.  As such, the track forecast envelope that a suite of models has come up with is similarly shifted.  That, of course, has implications throughout the next five days.

Tropical Storm Erika’s official track forecast continues to brush the storm against numerous islands over the next couple of days.  Antigua, Montserrat, Barbuda, Saint Martin, Saint Croix, and Saint Thomas are all in or near the expected path of this Tropical Storm.  Thereafter, T.S. Erika should pass to the northeast of San Juan, Puerto Rico Friday morning.  It is expected to continue moving northwest through Sunday, which would take the storm to the north of Hispanola, but over the Turks and Caicos Islands of the Bahamas.  Between Sunday and Monday, the official track forecast shifts the storm’s movement more to the NNW, keeping it to the east of other Bahama Islands.  That will likely take place Monday, all of which would be irrelevant if there is any short-term change to Erika’s path.

It should be noted that the official track forecast is on the northern portion of the model solution envelope.  Many models take Erika over Puerto Rico and Hispanola.  Interestingly, one model threads Erika by a series of islands, including between Cuba and Florida in the extended forecast.  I don’t buy this solution over any other – it’s just interesting.

The official intensity forecast is also interesting.  By Day 4, the NHC is calling for the storm to fall below Tropical Storm strength and continue along as a Tropical Depression instead.  This is because of the unexpected appearance of strong shear at upper-levels, which is making it difficult for Erika to establish outflow of rising air.  Compare this case to that of Bill, which developed excellent outflow early on, helping the system to intensify quickly.  A lot of factors have to come together at the same time for these systems to remain organized and intensify.

Elsewhere, the tropical wave that exited Africa over the weekend has dissipated.  No organized convection can currently be found where the disturbance was.  The next tropical wave transiting Africa is preparing to enter the far eastern Atlantic over the next day.  This wave is very vigorous – whether this continues over the ocean remains to be seen.  There is plenty of moist air surrounding this disturbance, but a rather intense shear environment is just to the disturbance’s north.

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