Hurricane ISAAC

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HURRICANE ISAAC DISCUSSION NUMBER  12
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
1100 AM EDT SAT SEP 30 2006

SATELLITE IMAGES INDICATE THAT ISAAC FORMED A RAGGED EYE SHORTLY
AFTER THE PREVIOUS ADVISORY.   A HIGH RESOLUTION QUIKSCAT PASS NEAR
1000 UTC SHOWED ONE BELIEVABLE WIND SPEED OF 63 KT.  HOWEVER THE
SATELLITE PRESENTATION HAS IMPROVED SOMEWHAT IN ORGANIZATION SINCE
THAT TIME.  AFTER BECOMING OBSCURED A FEW HOURS AGO... THE EYE HAS
BECOME A LITTLE BETTER-DEFINED IN VISIBLE IMAGERY.  IN ADDITION...
A SATELLITE ESTIMATE FROM TAFB IS AT T4.0... 65 KT... WHILE
CIMSS/CIRA AMSU ESTIMATES ARE BOTH ANALYZING ISAAC WITH 64 KT
WINDS. THEREFORE ISAAC IS UPGRADED TO A HURRICANE WITH 65 KT
WINDS... THE FIFTH HURRICANE OF THE SEASON. THE ENVIRONMENT LOOKS
CONDUCIVE FOR A LITTLE STRENGTHENING AS ISAAC REMAINS IN A
RELATIVELY LIGHT SHEAR AREA AND OVER SUFFICIENTLY WARM WATER
TEMPERATURES. HOWEVER... SOUTHWESTERLY SHEAR SHOULD BEGIN TOMORROW
DUE TO AN APPROACHING UPPER TROUGH AND LIMIT ANY FURTHER
INTENSIFICATION. THE INTENSITY FORECAST IS A BLEND OF THE FSU
SUPERENSEMBLE... GFDL AND THE PREVIOUS FORECAST. ISAAC IS EXPECTED
TO REMAIN A STRONG EXTRATROPICAL LOW BEFORE ITS EVENTUAL ABSORPTION
BY A LARGER LOW.
 
THE INITIAL MOTION ESTIMATE IS 300/6. A GRADUAL RIGHTWARD TURN
SHOULD OCCUR DURING THE NEXT 24 HOURS AS ISAAC ROUNDS THE WESTERN
PORTION OF THE SUBTROPICAL RIDGE. THEREAFTER THE CYCLONE SHOULD BE
ACCELERATING NORTHWARD AHEAD OF A DEEP MIDDLE-LATITUDE TROUGH OVER
THE WESTERN ATLANTIC OCEAN. MODEL GUIDANCE REMAINS IN GOOD
AGREEMENT AND THE OFFICIAL FORECAST IS BASICALLY AN UPDATE OF THE
PREVIOUS ONE. ISAAC SHOULD ENCOUNTER A MORE BAROCLINIC ENVIRONMENT
IN 36-48 HOURS... WHICH IS EXPECTED TO CAUSE IT TO TRANSITION INTO
AN EXTRATROPICAL CYCLONE BY 3 DAYS. GLOBAL MODELS ARE FORECASTING
THE REMNANTS OF ISAAC TO BE ABSORBED BY A LARGER EXTRATROPICAL LOW
NEAR THE 96 HOUR TIME FRAME. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      30/1500Z 30.9N  58.7W    65 KT
 12HR VT     01/0000Z 31.6N  59.6W    70 KT
 24HR VT     01/1200Z 33.4N  60.5W    75 KT
 36HR VT     02/0000Z 36.8N  60.5W    70 KT
 48HR VT     02/1200Z 41.5N  59.0W    60 KT...EXTRATROPICAL
 72HR VT     03/1200Z 50.0N  50.0W    50 KT...EXTRATROPICAL
 96HR VT     04/1200Z...ABSORBED
 
$$
FORECASTER BLAKE/STEWART
 
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