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HURRICANE ISAAC DISCUSSION NUMBER  12
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT SUN SEP 24 2000
 
THE SATELLITE PRESENTATION HAS DETERIORATED OVERNIGHT...WITH THE EYE
BECOMING LESS WELL-DEFINED AND THE CORE CONVECTION BECOMING LESS
SYMMETRIC.  SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN
102 KT...AND THE OFFICIAL INTENSITY REMAINS 105 KT FOR NOW.  A TRMM
OVERPASS AT 0503Z STILL SHOWED A VERY TIGHT EYEWALL BUT IT APPEARS
THAT ISAAC HAS PEAKED.
 
THE INITIAL MOTION IS 290/08.  THE FORECAST IS RELATIVELY
UNCOMPLICATED...AS ISAAC REMAINS SOUTH OF A MID-TROPOSPHERIC RIDGE
THAT SHOULD MAINTAIN THE CYCLONE ON A BASIC WEST-NORTHWESTERLY TRACK
FOR THE NEXT THREE DAYS.  THE 00Z AVN AND THE GUIDANCE DERIVED FROM
IT ARE GENERALLY SLOWER THAN THE UKMET.  WATER VAPOR IMAGERY SHOWS A
MID- TO UPPER-LEVEL LOW NEAR 25N53W.  THE AVN HAS A STRONGER
REFLECTION OF THIS FEATURE IN THE MID LEVELS THAN THE UKMET...AND IT
CONTRIBUTES TO A WEAKENING OF THE MID-TROPOSPHERIC RIDGE...HENCE THE
SLOWER MOTION.  THE OFFICIAL FORECAST IS ADJUSTED A LITTLE NORTH OF
THE PREVIOUS TRACK BUT REMAINS SOUTH OF MOST OF THE GUIDANCE.
 
SOME SHORT-TERM WEAKENING WILL OCCUR IF THE TIGHT EYEWALL IS IN FACT
COLLAPSING...BUT CONDITIONS SHOULD BE GENERALLY FAVORABLE FOR AT
LEAST THE NEXT DAY OR TWO. 
 
FORECASTER FRANKLIN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     24/0900Z 15.2N  36.1W   105 KTS
12HR VT     24/1800Z 15.6N  37.3W   100 KTS
24HR VT     25/0600Z 16.2N  38.9W   105 KTS
36HR VT     25/1800Z 16.8N  40.4W   105 KTS
48HR VT     26/0600Z 17.5N  42.0W   100 KTS
72HR VT     27/0600Z 19.0N  45.5W    95 KTS
 
 
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