10 August 2012

TD7 forms, TD8 on the way?

With Ernesto dissipating over the mountains of southern Mexico (producing a LOT of rain), I will focus on the two systems in the eastern Atlantic.

First, the easterly wave that left Africa on August 5 was upgraded to Tropical Depression 7 at 21Z (5pm EDT) yesterday.  The visible satellite image below is from today at 7:45am EDT when the Depression was centered about 800 miles east of the Lesser Antilles.  It's traveling west at a speedy 17kts.

It is forecast to remain fairly weak through the next 5 days due to a combination of vertical shear and dry air (sounds a lot like Ernesto!).  But, on its expected trajectory, it should cross over the Lesser Antilles late Saturday into Sunday morning, then reach Jamaica (the general vicinity, not necessarily a landfall on the island) by Tuesday afternoon.  Track guidance is shifting gradually to the north, which could bring it closer to Hispaniola rather than the open Caribbean -- that would have a huge impact on the intensity. 

The plots below show the intensity, track ,and some environmental parameters (shear, SST, RH) from key models.

Now on to the system that I've been mentioning over Africa.  It finally exited the coast on Thursday night, and model guidance was triggered immediately for it.  The wave actually has a couple of embedded Lows, one up near 16N 20W and the other further south near 9N 20W.  This is quite remarkable!  The primary 1004mb Low is the one to the north, but the one further south seems to be organizing as well.  The image here shows the infrared satellite image as the gray background, then ship observations are plotted on top, and the line contours basically show low-level rotation... you can see the broad-scale circulation, but also a separate one down near 9N.  I won't rely much on model guidance for this yet until we know which center will be the key player.

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