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HURRICANE ISAAC DISCUSSION NUMBER  31
NATIONAL WEATHER SERVICE MIAMI FL
11 PM EDT THU SEP 28 2000
 
HURRICANE ISAAC HAS WEAKENED SLIGHTLY BASED ON A SATELLITE INTENSITY
ESTIMATE OF 115 KT FROM ALL THREE SATELLITE AGENCIES AND A 28/2132Z
SSMI OVERPASS INDICATING CONCENTRIC EYEWALLS.  TOPS HAVE ALSO WARMED
DURING THE PAST 3 HOURS...SO ISAAC PROBABLY PEAKED AT 18Z AND 00Z. 
OUTFLOW REMAINS EXCELLENT AND THE OVERALL CONVECTIVE PATTERN HAS THE
CLASSIC CIRCULAR APPEARANCE.

INITIAL MOTION IS 345/15.  THERE IS NO SIGNIFICANT CHANGE TO THE
PREVIOUS FORECAST TRACK OR REASONING.  ISAAC APPEARS TO HAVE SLOWED
IN FORWARD SPEED...WHICH IS TYPICAL OF A TROPICAL CYCLONE AS IT
APPROACHES THE SUBTROPICAL RIDGE AXIS AND POINT OF RECURVATURE.
ISAAC MAY SLOW EVEN A LITTLE MORE BEFORE REACHING THE RIDGE AXIS
THAT LIES ALONG 32N-33N LATITUDE...AFTER WHICH GRADUAL ACCELERATION
IS FORECAST.  ALL OF THE GLOBAL MODELS AGREE ON ISAAC RECURVING TO
THE NORTHEAST IN ABOUT 24 HOURS.  THE ONLY OUTLIER IS THE UKMET
WHICH SLOWS THE FORWARD SPEED OF ISAAC BY DIGGING THE SYSTEM INTO
THE MEAN RIDGE AXIS RATHER THAN RIDING IT UP AND OVER THE RIDGE. THE
MAJORITY OF THE NHC MODEL GUIDANCE RECURVES ISAAC TO THE NORTHEAST
AND THAT SCENARIO SEEMS THE MOST REASONABLE GIVEN THE STRENGTH OF
THE APPROACHING EASTERN U.S. TROUGH.  THE OFFICIAL TRACK IS CLOSE TO
THE PREVIOUS FORECAST THROUGH 48 HOURS AND THEN SLIGHTLY TO THE
RIGHT BY 72 HOURS...WHICH IS SIMILAR TO THE GFDL AND BAMD MODELS.
 
NOW THAT ISAAC HAS LIKELY PEAKED AND IS UNDERGOING AN EYEWALL
REPLACEMENT CYCLE...SOME WEAKENING SHOULD OCCUR.  WEAKENING IS ALSO
LIKELY SINCE ISAAC IS FORECAST TO CONTINUE MOVING OVER COOLER SSTS. 
HOWEVER...GIVEN THE IMPRESSIVE OUTFLOW PATTERN...ONLY GRADUAL SLOW
WEAKENING IS FORECAST THROUGH 36 HOURS...AFTER WHICH MORE RAPID
WEAKENING IS EXPECTED AS ISAAC TRACKS OVER 24C TO 22C WATER.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     29/0300Z 28.6N  55.3W   115 KTS
12HR VT     29/1200Z 30.7N  56.0W   115 KTS
24HR VT     30/0000Z 33.3N  55.8W   110 KTS
36HR VT     30/1200Z 35.8N  54.1W   105 KTS
48HR VT     01/0000Z 38.0N  51.5W    95 KTS
72HR VT     02/0000Z 42.5N  44.5W    75 KTS
 
 
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