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Posted at 11:40 AM ET, 10/29/2010

Shary & friends keep tropical storm season alive

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One, two, three. Count 'em. We have three more tropical systems in the Atlantic Basin to monitor.

But are they really tropical? The two swirlies in the central Atlantic, roughly along the latitudes of Florida - one of which was named Tropical Storm Shary last night - are questionably so.

The system near South America (tagged as 91L by the National Hurricane Center), however, most definitely is. 91L is a large swirling disturbance that moved westward across the tropical Atlantic as an African Easterly Wave (AEW) - a kind of circulation that is generally quite capable of maturing into a tropical cyclone.

System #1, the cloud cluster located about 225 miles south southwest of Bermuda, designated as tropical storm Shary - with winds estimated at 40 mph - is a glorified complex of thunderstorms associated with a much larger upper-level low pressure system. A tropical storm warning is in effect for Bermuda where Shary may drop 1-3" of rain and generate some strong winds. Otherwise, it is not a threat to land on its projected track.

System #2, designated as 90L, consists of a somewhat concentrated group of thunderstorms roughly fifteen hundred miles to Shary's east, at about 25N and 42W.  It appears to be associated with a circulation that developed along the tail end of a cold front. Like Shary, it exhibits an irregular shape in the satellite imagery.  Strong winds aloft are likely the source of its disfiguration, and additional development is unlikely.

In a larger context, the circulations that encompass Shary and 90L are really remnants of the kind of mid-latitude weather systems (i.e., upper troughs and cold fronts) that bypass us every few days.

These particular circulations just happen to be located a bit further south, over 80-degree Atlantic waters, where their ability to trigger large thunderstorm clusters is facilitated by the semi-tropical climate. The windy and often dry atmosphere that defines these kinds of features in general, regardless of their location, makes them relatively poor candidates for significant tropical-cyclone development.

91L, on the other hand, is much more suited to evolve into a tropical cyclone. As suggested by its highly symmetrical, almost galactic, presentation in the satellite pictures, 91L is located in a much more humid environment that is somewhat divorced from the destructive influences of wind shear. This system should have a relatively high chance of becoming a depression during the next couple of days (or sooner), unless it crosses over South America.

The track guidance generally moves 91L northwestward over the Caribbean Sea early next week.

Prospects for its longer term survival, particularly if it continues northward from there, are not good however. The global weather models are now converging on a solution that closes off a large and very windy upper vortex over the Southern United States later next week. That could very well trigger a rapid demise of whatever becomes of 91L.

By  |  11:40 AM ET, 10/29/2010

Categories:  Tropical Weather

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