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TROPICAL STORM ISELLE DISCUSSION NUMBER  14
NATIONAL WEATHER SERVICE MIAMI FL
2 AM PDT WED SEP 18 2002
 
ISELLE CONTINUES TO WEAKEN AS INDICATED BY LATEST SATELLITE IMAGERY
WHICH SHOWS VIRTUALLY NO DEEP CONVECTION NEAR THE CIRCULATION
CENTER.  DVORAK INTENSITY ESTIMATES HAVE COME DOWN OVER THE PAST 6
HOURS...WITH TAFB AND SAB AT 3.5...55 KT...AND AFWA AT 2.0...30 KT.
INITIAL INTENSITY WILL BE LOWERED TO 45 KT FOR THIS FORECAST
PACKAGE.  ISELLE IS FORECAST TO CONTINUE WEAKENING THROUGH THE
FORECAST PERIOD...ALTHOUGH NOT AS FAST AS SHIPS IS INDICATING.
MARGINALLY WARM SEA SURFACE TEMPERATURES AND THE LACK OF SHEAR MAY
DELAY WEAKENING FOR THE FIRST 24 HOURS OR SO...HOWEVER BY 36 HOURS
INCREASING WESTERLY SHEAR AND COOLER WATER SHOULD BRING ABOUT STEADY
WEAKENING THROUGH DISSIPATION AT THE END OF THE FORECAST.

THE INITIAL MOTION ESTIMATE IS 315/04...BASED ON AN EXPECTED SLOW 
NORTHWARD TURN DURING THE NEXT 12 HOURS...ALTHOUGH THE PAST 12 HOUR 
MOTION HAS BEEN 300/4.  ISELLE APPEARS TO BE STUCK IN A WEAK 
STEERING FLOW ASSOCIATED WITH A BREAK IN THE SUBTROPICAL RIDGE...AND 
THIS PATTERN IS LIKELY TO CHANGE LITTLE THROUGH THE FORECAST PERIOD. 
AS OFTEN OCCURS IN WEAK STEERING PATTERNS...NHC MODEL TRACK GUIDANCE 
IS QUITE DIVERGENT.  CLIPPER AND THE BAM MODELS KEEP ISELLE ON A 
WEST-NORTHWEST TRACK THROUGH 72 HOURS...WHILE MOST OF THE DYNAMICAL 
MODELS TURN THE CYCLONE SLOWLY NORTHEAST FOR THE FIRST 24 HOURS OR 
SO...THEN BACK TO THE NORTH AND NORTHWEST TOWARD THE END OF THE 
FORECAST.  THE OFFICIAL FORECAST WILL KEEP A SLOW NORTHWARD MOTION 
THROUGH 36 HOURS WITH A GRADUAL TURN TO THE NORTHWEST IN THE LATTER 
PART OF THE FORECAST.  THIS IS TO THE LEFT OF THE GFDL-UKMET-NOGAPS 
MODEL CONSENSUS...BUT CLOSE TO THE NCEP GLOBAL MODEL AND WELL TO THE 
RIGHT OF THE BAM MODELS.  
 
FORECASTER STEWART/MOLLEDA
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     18/0900Z 22.3N 114.9W    45 KTS
12HR VT     18/1800Z 22.8N 115.1W    40 KTS
24HR VT     19/0600Z 23.5N 115.0W    35 KTS
36HR VT     19/1800Z 24.2N 115.0W    35 KTS
48HR VT     20/0600Z 25.0N 115.5W    30 KTS
72HR VT     21/0600Z 26.0N 116.5W    25 KTS...DISSIPATING
 
 
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