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HURRICANE ISAAC DISCUSSION NUMBER  21
NATIONAL WEATHER SERVICE MIAMI FL
11 AM EDT TUE SEP 26 2000

AS WAS SUSPECTED EARLIER...SOUTHWESTERLY SHEAR IS HAVING A GREATER
IMPACT ON ISAAC THIS MORNING.  THIS IS APPARENTLY DUE TO A LITTLE
STRONGER EAST-SOUTHEASTERLY LOW-LEVEL FLOW RATHER THAN STRONGER
UPPER-LEVEL WESTERLIES.  CURRENT INTENSITY IS REDUCED TO 80
KNOTS...AND A LITTLE MORE WEAKENING MAY OCCUR DUE TO THE CURRENT
SHEARING ENVIRONMENT.  THE INTENSITY TRENDS SHOWN BELOW ARE SIMILAR
TO THOSE GIVEN BY THE LATEST SHIPS GUIDANCE.  LATER IN THE
PERIOD...AS ISAAC TURNS MORE TO THE RIGHT AND SEA SURFACE
TEMPERATURES BECOME A LITTLE WARMER...RESTRENGTHENING MAY OCCUR.

AS A RESULT OF THE SHEAR...THE CENTER IS LOCATED A LITTLE TO THE
SOUTHWEST OF THE PREVIOUS TRACK.  INITIAL MOTION IS ADJUSTED
SLIGHTLY MORE TO THE WEST...290/10.  THE FORECAST TRACK IS ALSO
SLIGHTLY TO THE LEFT OF THAT FROM THE PREVIOUS ADVISORIES FOR THE
FIRST HALF OF THE FORECAST PERIOD...BUT IS PRETTY MUCH THE SAME
THEREAFTER.  ISAAC SHOULD GRADUALLY TURN TOWARD THE NORTHWEST AND
INCREASE ITS FORWARD SPEED IN RESPONSE TO AN INCREASED STEERING FLOW
BETWEEN THE SUBTROPICAL HIGH AND A BROAD TROUGH OVER THE EXTREME
WESTERN ATLANTIC DURING THE NEXT FEW DAYS.  THE OFFICIAL FORECAST
TRACK IS ROUGHLY A CONSENSUS OF ALL OBJECTIVE GUIDANCE TRACKS...
WHICH REMAIN IN FAIRLY CLOSE AGREEMENT ON HEADING BUT WITH SOME
VARIANCE OF FORWARD SPEEDS.
 
FORECASTER PASCH
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     26/1500Z 18.8N  44.4W    80 KTS
12HR VT     27/0000Z 19.3N  45.9W    75 KTS
24HR VT     27/1200Z 20.5N  48.0W    75 KTS
36HR VT     28/0000Z 22.0N  50.0W    80 KTS
48HR VT     28/1200Z 23.5N  52.0W    85 KTS
72HR VT     29/1200Z 27.5N  56.5W    85 KTS
 
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