ZCZC MIATCDAT3 ALL 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 NNNN
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Page last modified: Monday, 07-Feb-2005 16:49:58 UTC