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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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