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Subsidence Covers Much of Atlantic Basin

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Updated: 7/18/2013 8:56 am
A rather large upper level low is over over the Gulf of Mexico helping to trigger numerous -- but disorganized -- clusters of heavy showers & storms from the Eastern Gulf across Fl. into the Bahamas.  This upper level feature will hold stationary or move slowly west/northwest but with no surface development expected.

Lots of convection across S. Fl. in association with the upper low as can be seen below (radar imagery courtesy S. Fl. Water Management District):

The satellite image below shows a very large area of stable air & cool temps. aloft covering much of the Central Atlantic east of the Caribbean - notice the light gray, "dotted" appearance of the cloud cover on the infrared level.  The second image below -- water vapor satellite -- shows this stable area very nicely (black & rust-colored area). Such an atmosphere is generally unfavorable for tropical development.

A weak tropical wave has entered the Eastern Caribbean.  This wave came off the coast of Africa late last week & has been slowly moving west.  Convection has increased some but models are not enthusiastic regarding development.  The water vapor image below gives us a hint as to why: a good deal of dry air is over the Caribbean along with significant shear.  If the wave becomes something to reckon with, it would be in the long term over the Western Caribbean or Gulf though virtually no forecast models indicate such development at the moment.  

Meanwhile....tropical waves moving off of Africa are weak & showing no signs strengthening.

You will often see me refer to potential velocity anomalies when discussing the tropics -- see the map below.  The brown lines -- which cover virtually all of the E. Pacific & Atlantic Basin -- represent -- in simplest terms -- sinking (or subsident) air in the atmosphere which is not generally conducive to the development of tropical cyclones.  The green areas represent upward motion & leads to more convection & is much more favorable for tropical cyclone development.  Notice in the IR imagery which is overlaid in the image below that there is much more convection in the green areas as evidenced by the colder cloud tops (thunderstorms).  These upward vertical velocities should spread east over the next couple weeks, & we should see the first signs of this with tropical development in the E. Pacific the last 10 days of this month followed by the upward velocities spreading east into the Caribbean & Gulf & the rest of the Atlantic Basin where tropical cyclone development will then become more likely -- approximately the last few days of July & the first week of Aug.  Indeed....the GFS forecast model is showing some long term development from an African wave in about 10-14 days.

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