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TROPICAL STORM ISELLE DISCUSSION NUMBER  13
NATIONAL WEATHER SERVICE MIAMI FL
8 PM PDT TUE SEP 17 2002
 
ISELLE IS NOT LOOKING VERY HEALTHY ON INFRARED SATELLITE IMAGERY. 
WATER VAPOR IMAGERY INDICATES SOME DRY AIR IS PROBABLY BEING 
ENTRAINED INTO THE CYCLONE FROM THE SOUTHEAST. UPPER-LEVEL OUTFLOW 
REMAINS FAIR TO GOOD IN ALL QUADRANTS DESPITE THE SIGNIFICANT 
WARMING OF THE CLOUD TOPS DURING THE PAST 3 HOURS.
 
THE INITIAL MOTION ESTIMATE IS 310/04. THIS MOTION IS BASED ON AN 
EXPECTED SLOW NORTHWARD TURN DURING THE NEXT 6 TO 12 HOURS. 
HOWEVER...THE PAST 6 HOUR MOTION HAS BEEN 300/06. AFTER 12 HOURS... 
MOST OF THE MODELS AGREE ON A RETURN TO A GENERAL SLOW NORTHWEST OR 
WEST-NORTHWEST MOTION. A BREAK IN THE SUBTROPICAL RIDGE MAY NOT BE 
STRONG ENOUGH FOR A WEAKENING ISELLE TO PASS THROUGH AND MOVE TO THE 
NORTHEAST LIKE THE DEEP BAM AND LBAR MODELS ARE SUGGESTING. THERE IS 
ALSO MUCH MORE DIVERGENCE IN THE NHC MODEL GUIDANCE THUS CYCLE WITH 
TRACKS IN ALL DIRECTIONS EXCEPT TO THE SOUTH. AS SUCH...I HAVE KEPT 
THE FORWARD SPEED SLOW AND THE OFFICIAL FORECAST TRACK IS CLOSE TO 
THE AVN-NOGAPS-UKMET CONSENSUS.

ISELLE HAS PROBABLY PEAKED UNLESS A BRIEF BURST OF DEEP CONVECTION 
RETURNS LATER TONIGHT DURING THE CONVECTIVE MAXIMUM PERIOD. OUTFLOW 
IS GOOD AND SSTS ARE WARM ENOUGH TO SUPPORT A MINIMAL HURRICANE LIKE 
THR SHIPS INTENSITY MODEL IS FORECASTING. HOWEVER...THE 
AFOREMENTIONED MID-LEVEL DRY INTRUSION WILL LIKELY PREVENT ANY 
SUSTAINED CONVECTIVE DEVELOPMENT FROM OCCURRING...EVEN THOUGH THE 
SHEAR IS EXPECTED TO REMAIN BELOW 10 KT FOR THE NEXT 30 HOURS OR SO. 
BY 36 HOURS... INCREASING SOUTHWESTERLY SHEAR AND SSTS BELOW 25C 
SHOULD BRING ABOUT STEADY WEAKENING LIKE SHIPS IS INDICATING.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     18/0300Z 22.0N 114.7W    55 KTS
12HR VT     18/1200Z 22.4N 115.1W    55 KTS
24HR VT     19/0000Z 23.1N 115.2W    55 KTS
36HR VT     19/1200Z 23.4N 115.7W    55 KTS
48HR VT     20/0000Z 23.8N 116.5W    50 KTS
72HR VT     21/0000Z 25.0N 117.5W    40 KTS
 
 
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