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HURRICANE ISAAC DISCUSSION NUMBER  13
NATIONAL WEATHER SERVICE MIAMI FL
11 AM EDT SUN SEP 24 2000

AFTER RAPIDLY STRENGTHENING TO CATEGORY THREE STATUS...ISAAC HAS
WEAKENED THIS MORNING.  THIS WAS APPARENTLY DUE TO AN INNER CORE
FLUCTUATION SINCE THE LARGE-SCALE ENVIRONMENT STILL APPEARS TO BE
FAVORABLE.  THE OFFICIAL INTENSITY FORECAST SHOWS ONLY A SLIGHT
FLUCTUATION...WITH SOME WEAKENING LATER IN THE PERIOD SINCE ISAAC IS
EXPECTED TO CROSS OVER THE SLIGHTLY COOLER WATERS NEAR 43-45 W
LONGITUDE.  WESTERLY SHEAR MAY ALSO INCREASE OVER THE HURRICANE IN A
COUPLE OF DAYS.  THE UPPER-LEVEL LOW PREVIOUSLY MENTIONED NEAR
25N53W APPEARS TO HAVE WEAKENED...BUT THE NCEP GLOBAL MODEL
PREDICTS A NEW UPPER LOW TO DROP INTO ABOUT THE SAME PLACE IN THE
LATTER PART OF THE FORECAST PERIOD...WHICH WOULD INCREASE THE SHEAR.

ON THE OTHER HAND THE U.K. MET OFFICE MODEL RUN KEEPS A FAVORABLE
UPPER-LEVEL ENVIRONMENT THROUGH THE PERIOD.  THE LATEST SHIPS MODEL
OUTPUT CALLS FOR GREATER WEAKENING THAN I HAVE INDICATED HERE...BUT
IT SEEMS TO BE EXAGGERATING THE INFLUENCE OF LOWER SST.

MOTION CONTINUES WNW AT A SLIGHTLY FASTER FORWARD SPEED...290/10.   
ISAAC IS EXPECTED TO REMAIN SOUTH OF THE SUBTROPICAL RIDGE FOR THE
NEXT FEW DAYS...AND THEREFORE CONTINUE ITS WNW MOTION. THE CURRENT
OFFICIAL TRACK FORECAST IS AN EXTENSION OF THE PREVIOUS ONE...AND IS
ABOUT IN THE MIDDLE OF THE GUIDANCE SUITE.  IT IS ALSO VERY SIMILAR
TO THE LATEST RUN OF THE EXPERIMENTAL FSU SUPERENSEMBLE MODEL.

THE RADII OF TROPICAL STORM FORCE WINDS HAVE BEEN INCREASED SLIGHTLY
ON THE BASIS OF A RECENT QUIKSCAT PASS.
 
FORECASTER PASCH
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     24/1500Z 15.7N  37.4W    95 KTS
12HR VT     25/0000Z 16.2N  38.8W   100 KTS
24HR VT     25/1200Z 16.8N  40.7W   100 KTS
36HR VT     26/0000Z 17.5N  42.5W   100 KTS
48HR VT     26/1200Z 18.5N  44.5W    95 KTS
72HR VT     27/1200Z 20.0N  48.0W    90 KTS
 
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