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TROPICAL STORM ISAAC DISCUSSION NUMBER   8
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT SAT SEP 23 2000
 
METEOSAT IR SATELLITE IMAGES SHOW THAT THE CLOUD PATTERN CONTINUES
TO IMPROVE GRADUALLY AND IT HAS BEEN CHANGING FROM BANDING TO CDO. 
THE OUTFLOW AT THIS TIME LOOKS GOOD WITH UPPER-LEVEL CLOUDS
EMANATING FROM THE CENTER TOWARD THE WEST AND NORTHWEST AT HIGH
SPEED.  THIS IS IN GENERAL A SIGN OF STRENGTHENING.  T-NUMBERS ARE
3.5 FROM BOTH AGENCIES SUGGESTING THAT THE INITIAL INTENSITY IS AT
LEAST 55 KNOTS.  

HOWEVER...DESPITE THE CURRENT FAVORABLE CONDITIONS...I SEE AN
UNFAVORABLE ENVIRONMENT AHEAD OF ISAAC.  NCEP GLOBAL AND NOGAPS
MODELS DEVELOP A LARGE UPPER-LEVEL ANTICYCLONE WELL AHEAD OF THE
TROPICAL CYCLONE WITH A TROUGH DIGGING SOUTHWARD. THE ANTICYCLONE IS
ALREADY BEING DEPICTED ON WV IMAGES. THE UK MODEL BRINGS UPPER-LEVEL
NORTHWESTERLY FLOW OVER THE TROPICAL CYCLONE. THIS WOULD PLACE ISAAC
WITHIN A SHEARING ENVIRONMENT IN THREE DAYS UNLESS THE TROPICAL
CYCLONE MOVES FASTER AND REMAINS UNDER THE UPPER-LEVEL ANTICYCLONE. 
IT SEEMS THAT ISAAC IS DOING THE OPPOSITE...ITS IS SLOWING DOWN
INSTEAD.  GIVEN THE HOSTILE ENVIRONMENT THAT HAS PREVAILED THIS
SEASON...THE OPTION OF SHEARING IS THE MOST LIKELY TO OCCUR. 
NEVERTHELESS...I AM NOT CONFIDENT ENOUGH TO MAKE A CHANGE FROM THE
PREVIOUS INTENSITY FORECAST AT THIS TIME.  

BEST ESTIMATE OF THE INITIAL MOTION IS 280/10 OR SLOWER. THERE IS
ENOUGH RIDGING TO THE NORTH TO KEEP THE TROPICAL CYCLONE ON A SLOW
WESTWARD TRACK THROUGH 72 HOURS.  THERE ARE SEVERAL MODELS WHICH
KEEP ISAAC ON A FASTER WESTWARD TRACK BUT THE OFFICIAL FORECAST
FOLLOWS THE GFDL VERY CLOSELY.  THE LONG RANGE INTEGRATION OF THIS
MODEL BRINGS ISAAC FARTHER TO WEST THAN THE TWO PREVIOUS RUNS BUT
STILL NORTH OF THE LEEWARD ISLANDS. 
 
FORECASTER AVILA
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     23/0900Z 14.0N  32.8W    55 KTS
12HR VT     23/1800Z 14.1N  34.3W    60 KTS
24HR VT     24/0600Z 14.2N  36.3W    70 KTS
36HR VT     24/1800Z 14.5N  38.0W    75 KTS
48HR VT     25/0600Z 15.0N  40.0W    80 KTS
72HR VT     26/0600Z 15.5N  43.5W    80 KTS
 
 
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