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Tropical Storm ISABEL


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TTAA00 KNHC DDHHMM
TROPICAL STORM ISABEL DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 PM EDT SAT SEP 06 2003

SATELLITE IMAGERY DURING THE EVENING SHOWS THAT ISABEL HAS A
WELL-DEFINED CURVED CONVECTIVE BAND WITH TOPS COLDER THAN -80C. 
THE CIRRUS OUTFLOW IS EXCELLENT OVER THE SOUTH SEMICIRCLE AND FAIR
TO GOOD ELSEWHERE.  SATELLITE INTENSITY ESTIMATES ARE 55 KT FROM
TAFB AND SAB...AND 45 KT FROM AFWA.  THE INITIAL INTENSITY IS
RAISED TO 50 KT FOR THIS ADVISORY.

THE INITIAL MOTION IS 270/09.  WATER VAPOR IMAGERY INDICATES THAT
ISABEL IS SOUTH OF A MID/UPPER-LEVEL RIDGE...WITH AN UPPER-LEVEL
LOW CENTERED NEAR 26N46W.  LARGE-SCALE MODELS SUGGEST THAT THIS LOW
WILL CREATE ENOUGH WEAKNESS IN THE RIDGE TO ALLOW ISABEL TO
GRADUALLY TURN TO A WEST-NORTHWESTWARD COURSE DURING THE NEXT 24-48
HR...AND CONTINUE THAT GENERAL MOTION THROUGH DAY 5.  NHC TRACK
GUIDANCE AGREES WITH THIS SCENARIO.  SINCE SOME MODELS SHOWED
SOMEWHAT OF A NORTHWARD BIAS WHEN FABIAN WAS IN THIS AREA...THE
OFFICIAL FORECAST WILL LEAN TOWARD THE SOUTH SIDE OF THE GUIDANCE. 
IT IS ALSO NUDGED JUST A LITTLE SOUTH OF THE PREVIOUS TRACK.

ISABEL SEEMS TO HAVE ALL THE INGREDIENTS TO STRENGTHEN
SIGNIFICANTLY...A LARGE CYCLONIC ENVELOPE...GOOD CONVECTIVE
STRUCTURE...COLD CLOUD TOPS...AND FAVORABLE OUTFLOW AND VERTICAL
SHEAR.  THE SHIPS MODEL TAKES THE STORM TO 80 KT IN 60 HR AND THEN
STOPS INTENSIFICATION FOR REASONS NOT READILY APPARENT.  THE GFDL
MAKES ISABEL A 90 KT HURRICANE IN 120 HR.  THE INTENSITY FORECAST
WILL SHOW THE INTENSITY LEVELING OFF AT 80 KT IN AGREEMENT WITH THE
SHIPS MODEL.  HOWEVER...IT WOULD NOT BE A SURPRISE TO SEE ISABEL
STRENGTHEN FASTER AND MORE THAN CURRENTLY FORECAST.
 
FORECASTER BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      07/0300Z 13.4N  36.1W    50 KT
 12HR VT     07/1200Z 13.4N  37.4W    60 KT
 24HR VT     08/0000Z 13.8N  39.4W    65 KT
 36HR VT     08/1200Z 14.7N  41.8W    70 KT
 48HR VT     09/0000Z 15.6N  44.4W    75 KT
 72HR VT     10/0000Z 17.0N  49.4W    80 KT
 96HR VT     11/0000Z 18.0N  54.5W    80 KT
120HR VT     12/0000Z 18.5N  59.0W    80 KT
 
 
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