Tropical Storm ISIS

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TROPICAL STORM ISIS DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
8 PM PDT WED SEP 08 2004
 
THE REASONING FOR THE TRACK FORECAST HAS NOT DEVIATED FROM THE
PREVIOUS FORECAST CYCLE...EXCEPT THAT THE INITIAL POSITION HAS BEEN
ADJUSTED TO THE NORTH AND EAST AS THE LOW LEVEL CIRCULATION BECAME
EXPOSED. THE CENTER WAS EASILY TRACKED FOR SEVERAL HOURS TO GIVE AN
INITIAL MOTION OF 335 DEGREES AT 8 KT...A BIT FASTER THAN THE
PREVIOUS MOTION. ISIS HAS BEEN EXPERIENCING RATHER SEVERE EASTERLY
SHEAR THROUGHOUT THE DAY TODAY WITH THE MAIN CONVECTIVE AREA STILL
LOCATED WELL WEST OF THE LOW LEVEL CIRCULATION. THERE WAS SOME
INDICATION IN THE LAST FEW VISIBLE SATELLITE FRAMES THAT NEW
CONVECTION WAS DEVELOPING NEAR THE LOW LEVEL VORTEX. CLARIFICATION
OF THE ACTUAL CENTER POSITION AND MOTION PROVIDES SUFFICIENT
EVIDENCE THAT ISIS WILL EXIT THE HIGH VERTICAL SHEAR ENVIRONMENT A
BIT EARLIER THAN ANTICIPATED JUST A FEW HOURS AGO. SEA SURFACE
TEMPERATURES ALONG THE FORECAST TRACK WILL REMAIN AT LEAST 28 DEG C
THROUGH 48 HOURS...AND VERTICAL SHEAR IS FORECASTED TO DECREASE.
INTENSITY GUIDANCE IS MORE IN LINE AS THE GFDL MODEL HAS SUDDENLY
TAKEN A LIKING TO ISIS. THE SHIPS MODEL ALSO FORECASTS
STRENGTHENING AND IS NOW MORE CONSERVATIVE THAN THE GFDL. BASED ON
THIS GUIDANCE AND THE MORE NORTHERLY INITIAL MOTION...THE INTENSITY
FORECAST IS SLIGHTLY MORE AGGRESSIVE THAN THE PREVIOUS FORECAST.

THE TRACK FORECAST IS FAIRLY STRAIGHT FORWARD.  ISIS WILL BEGIN TO
SLOW DOWN AND TAKE A GRADUAL TURN TOWARD THE NORTHWEST AND THEN
WEST-NORTHWEST AS IT ENCOUNTERS A STRONG SUB-TROPICAL RIDGE TO THE
NORTH.  AN INCREASE IN FORWARD SPEED SHOULD OCCUR LATER IN THE
FORECAST PERIOD AS THE RIDGE STRENGTHENS.  ISIS SHOULD SLOWLY
WEAKEN AFTER THE 72 HOUR FORECAST PERIOD AS IT MOVES OVER SUB 26
DEG C WATER...REMAINING IN A WEAK VERTICAL SHEAR ENVIRONMENT.  

FORECASTER BEVEN/HENNON
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      09/0300Z 16.6N 111.5W    35 KT
 12HR VT     09/1200Z 17.0N 111.9W    40 KT
 24HR VT     10/0000Z 17.6N 113.5W    45 KT
 36HR VT     10/1200Z 18.1N 115.4W    50 KT
 48HR VT     11/0000Z 18.5N 118.0W    60 KT
 72HR VT     12/0000Z 19.3N 123.5W    55 KT
 96HR VT     13/0000Z 19.8N 129.4W    50 KT
120HR VT     14/0000Z 20.3N 136.1W    40 KT
 
 
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