Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER   9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
500 PM EDT FRI SEP 29 2006
 
CONVECTION HAS BEEN INTENSIFYING AND ATTEMPTING TO FORM A RING
AROUND THE CENTER OF ISAAC. IN ADDITION THE OUTFLOW HAS BEEN
EXPANDING AND BECOMING MORE SYMMETRIC. THESE ARE ALL SIGNS OF AN
INTENSIFYING SYSTEM. DVORAK T-NUMBERS HAVE INCREASED MARKEDLY...
AND SINCE AN EARLIER QUIKSCAT PASS SUPPORTED WINDS OF 40-45 KT
TWELVE HOURS AGO... THE INITIAL INTENSITY IS RAISED TO 50 KT. ISAAC
IS LEAVING THE COOLER WATERS PREVIOUSLY UPWELLED BY GORDON AND
HELENE AND MOVING INTO WARMER SSTS. IN ADDITION... THE DRY AIR IN
THE MIDDLE TO UPPER LEVELS NEAR THE SYSTEM'S CORE HAS BEEN REDUCED
AND THERE IS LITTLE SHEAR APPARENT ON SATELLITE IMAGES. THESE
FACTORS SUPPORT INTENSIFICATION FOR THE NEXT 36 HOURS OR SO. THUS
THE INTENSITY FORECAST IS RAISED FROM THE PREVIOUS ONE AND SHOWS
ISAAC BECOMING A HURRICANE. THE FORECAST IS HIGHER THAN THE
PREVIOUS ONE AND IN BEST AGREEMENT WITH A BLEND OF THE SHIPS/FSSE. 
 
THE INITIAL MOTION IS VIRTUALLY UNCHANGED FROM EARLIER AND IS ABOUT
295/6. MIDDLE-LEVEL RIDGING OVER THE CENTRAL ATLANTIC SHOULD BE THE
PRIMARY STEERING MECHANISM FOR THE NEXT ONE OR TWO DAYS... SENDING
THE STORM ON A WEST-NORTHWEST TO NORTHWESTWARD HEADING. THEREAFTER
ISSAC WILL REACH THE WESTERN PERIPHERY OF THE RIDGE AND LIKELY
ACCELERATE TO THE NORTH IN FRONT OF A MASSIVE TROUGH FORECAST TO
PROGRESS EASTWARD ACROSS THE WESTERN ATLANTIC. THE UKMET HAS
SHIFTED ITS TRACK WESTWARD FROM EARLIER BUT IS STILL ON THE EASTERN
SIDE OF THE MODEL ENVELOPE. OTHER MODELS ARE MOSTLY UNCHANGED OR
SLIGHTLY FARTHER WEST OF THE PREVIOUS GUIDANCE. THE OFFICIAL
FORECAST IS NUDGED WESTWARD BUT STILL LIES JUST EAST OF THE MODEL
CONSENSUS. GLOBAL MODELS SUGGEST THAT ISAAC WILL BECOME RAPIDLY
EXTRATROPICAL BETWEEN 48 AND 72 HOURS FOLLOWED BY ABSORPTION INTO A
LARGER EXTRATROPICAL LOW IN 4 TO 5 DAYS. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      29/2100Z 29.7N  56.8W    50 KT
 12HR VT     30/0600Z 30.1N  57.8W    55 KT
 24HR VT     30/1800Z 31.1N  59.1W    60 KT
 36HR VT     01/0600Z 32.5N  60.3W    65 KT
 48HR VT     01/1800Z 34.7N  61.0W    60 KT
 72HR VT     02/1800Z 43.5N  57.0W    55 KT...EXTRATROPICAL
 96HR VT     03/1800Z 50.0N  48.0W    50 KT...EXTRATROPICAL
120HR VT     04/1800Z...ABSORBED
 
$$
FORECASTER BLAKE/STEWART
 
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