ZCZC MIATCDAT3 ALL TTAA00 KNHC DDHHMM HURRICANE ISABEL DISCUSSION NUMBER 31 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 PM EDT SAT SEP 13 2003 THERE HAS BEEN A PLETHORA OF WIND DATA FROM BOTH THE AIR FORCE RESERVE AND NOAA HURRICANE HUNTERS THIS AFTERNOON. IN SUMMARY...THE AIRCRAFT FOUND MAXIMUM 700 MB FLIGHT-LEVEL WINDS OF 157 AND 158 KT ...ALONG WITH A DROPSONDE WIND VALUE OF 178 KT...OR 205 MPH...AT 879 MB...ALL OF WHICH CORRESPONDS TO SURFACE WIND ESTIMATES OF 141-142 KT. THEREFORE...THE INITIAL INTENSITY HAS BEEN INCREASED TO 140 KT/160 MPH...SO ISABEL IS A CATEGORY 5 HURRICANE ONCE AGAIN. THE INITIAL MOTION ESTIMATE IS NOW 285/11. THIS MAY BE A SHORT-TERM ACCELERATION DUE TO THE EYE CONTRACTING DOWN IN SIZE AND RE-ADJUSTING WITHIN THE CENTRAL DEEP CONVECTION. HOWEVER...THE 285 DEGREES MOTION NOW APPEARS TO BE A STEADY TREND. THE NEW 12Z GFS FORECAST TRACK HAS COME IN ALMOST ON TOP OF THE PREVIOUS OFFICAL FORECAST THROUGH 48 HOURS AND THEN IS LEFT OF THE TRACK BY 72 HOURS ...BEFORE HOOKING BACK ACROSS THE PREVIOUS FORECAST TRACK AND ACCELERATING ISABEL NORTHWARD. THIS SCENARIO IS ALSO IDENTICAL TO THE 12Z ETA MODEL RUN. SIMILARLY BUT TO THE RIGHT OF THE PREVIOUS FORECAST ARE THE GFDL...NOGAPS...UKMET...AND CANADIAN MODELS. INTERESTINGLY ENOUGH...ALL OF THE MODELS AGREE ON A SHARP DECREASE IN FORWARD SPEED OR A STALL IN 60-72 HOURS...AND THEN RAPIDLY ACCELERATING ISABEL NORTH OR NORTH-NORTHWESTWARD AFTER 96 HOURS AS THE CYCLONE IS FORECAST TO GET PICKED UP BY AN APPROACHING SHORTWAVE TROUGH FROM THE WEST AND A BUILDING RIDGE TO THE EAST. HOWEVER...THERE IS STILL SOME UNCERTAINTY ON THE CURRENT STRENGTH OF THE SUBTROPICAL RIDGE TO THE NORTH OF ISABEL...SO IT IS PRUDENT TO WAIT UNTIL ALL OF THE NOAA GULFSTREAM-IV DROPSONDE DATA MAKE IT INTO THE 14/00Z MODELS. THE OFFICIAL FORECAST TRACK IS JUST AN UPDATE OF THE PREVIOUS TRACK THROUGH 96 HOURS...WITH A LITTLE MORE NORTHWARD ACCELERATION INDICATED AT 120 HOURS AS A COURSE OF LEAST REGRET. NOTE -- NONE OF THE NHC MODEL GUIDANCE HAS ISABEL OVER LAND BY 120 HOURS. WHILE SOME SLIGHT STRENGTHENING IS POSSIBLE IN THE NEXT 12 HOURS... THERE IS NO WAY TO REALLY TIME THE DEVELOPMENT OF ANOTHER EYEWALL REPLACEMENT CYCLE. IN THE LONG RUN...UPPER-LEVEL CONDITIONS ARE FORECAST TO REMAIN FAVORABLE FOR AT LEAST THE NEXT 48-72 HOURS... WITH THE HURRICANE ALSO MOVING OVER WARMER WATER. THEREFORE...A STEADY INTENSITY OR ONLY VERY SLOW WEAKENING IS EXPECTED FOR THE NEXT COUPLE OF DAYS. BY 120 HOURS...ISABEL MAY BEGIN TO EXPERIENCING UPPER-LEVEL SOUTH TO SOUTHWESTERLY SHEAR...WHICH SHOULD BRING ABOUT MORE RAPID WEAKENING. HOWEVER...ISABEL IS STILL EXPECTED TO REMAIN A MAJOR HURRICANE THOUGHOUT THE FORECAST PERIOD. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 13/2100Z 22.6N 62.6W 140 KT 12HR VT 14/0600Z 23.0N 64.2W 140 KT 24HR VT 14/1800Z 23.7N 66.2W 140 KT 36HR VT 15/0600Z 24.4N 68.0W 135 KT 48HR VT 15/1800Z 25.2N 69.4W 135 KT 72HR VT 16/1800Z 26.5N 71.5W 130 KT 96HR VT 17/1800Z 29.5N 73.0W 125 KT 120HR VT 18/1800Z 33.5N 75.0W 105 KT NNNN
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