ZCZC MIATCDAT3 ALL TTAA00 KNHC DDHHMM HURRICANE ISABEL DISCUSSION NUMBER 28 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 11 PM EDT FRI SEP 12 2003 RADAR IMAGES TRANSMITTED FROM A NOAA/HURRICANE RESEARCH DIVISION FLIGHT INTO ISABEL LATE THIS AFTERNOON SHOWED THE DEVELOPMENT OF A SECOND CONCENTRIC EYEWALL ABOUT 20 NM OUTSIDE OF THE INNER EYEWALL. THE AIRCRAFT CREW ALSO REPORTED A STEPPED FREQUENCY MICROWAVE RADIOMETER MEASUREMENT OF A 140 KT SURFACE WIND LATE IN THE AFTERNOON. CONCURRENT WITH THIS RE-DEVELOPMENT OF AN OUTER EYEWALL...THE CORE CONVECTION IS CURRENTLY WEAKENING AS IT DID LAST NIGHT AT THIS TIME. IN ADDITION...MICROWAVE IMAGERY INDICATES THAT SOME DRY AIR HAS BEEN INFILTRATING THE CIRCULATION. SUBJECTIVE DVORAK T-NUMBERS ARE DOWN TO 6.0...AND THE LATEST CIMSS ADVANCED OBJECTIVE DVORAK 3-HR AVERAGE T NUMBER IS ALSO 6.0...WELL BELOW CATEGORY FIVE INTENSITY. I VERY MUCH DOUBT THAT ISABEL STILL HAS WINDS OF 140 KT...BUT SINCE ANOTHER AIRCRAFT WILL BE IN THE HURRICANE IN JUST THREE HOURS...WE WILL WAIT FOR CONFIRMATION OF THIS APPARENT WEAKENING TREND. IT IS VIRTUALLY UNHEARD OF FOR A HURRICANE TO REMAIN AT CATEGORY FIVE STRENGTH FOR MORE THAN 30 HOURS. HOWEVER...ISABEL IS EXPECTED TO REMAIN A MAJOR HURRICANE THROUGHOUT THE FORECAST PERIOD. THE INITIAL MOTION IS 275/8. ISABEL REMAINS SOUTH OF A DEEP-LAYER ANTICYCLONE THAT IS PROVIDING THE WESTWARD STEERING CURRENT. THIS ANTICYCLONE CONTINUES WESTWARD IN STEP WITH THE HURRICANE...AND MODEL GUIDANCE IS IN EXCELLENT AGREEMENT ON A WEST TO WEST-NORTHWEST TRACK FOR THE NEXT 72 HOURS. BY 72 HOURS...THE EVOLUTION OF A MID- TO UPPER-LEVEL LOW CURRENTLY OVER THE CAROLINAS COULD PLAY A KEY ROLE IN WHAT PORTION OF THE U.S. EAST COAST WOULD EVENTUALLY BE AFFECTED BY ISABEL. THE GFS AND NOGAPS MODELS ARE VERY SIMILAR IN HOLDING THIS FEATURE IN PLACE ALMOST A DAY LONGER THAN THE UKMET...WITH THE RESULT THAT THE FORMER TWO MODELS TURN ISABEL MUCH MORE SHARPLY NORTHWARD THAN THE UKMET. THE GFDL TURNS ISABEL EVEN MORE SHARPLY THAN THE GFS AND NOGAPS MODELS...AND THE EUROPEAN MODEL ALSO SEEMS TO FAVOR AN EARLIER TURN NORTH. THE OFFICIAL FORECAST HAS BEEN BEEN SHIFTED TO THE RIGHT OF THE PREVIOUS ADVISORY TRACK...BUT IS STILL CLOSER TO THE UKMET THAN THE OTHER GUIDANCE. IN PART THIS IS TO MAINTAIN CONTINUITY WITH PREVIOUS FORECASTS...BUT ALSO BECAUSE THE UKMET HAS BEEN PRESENTING A MORE CONSISTENT SOLUTION OVER THE PAST COUPLE OF DAYS THAN HAVE THE OTHER MODELS. FORECASTER FRANKLIN FORECAST POSITIONS AND MAX WINDS INITIAL 13/0300Z 21.8N 59.5W 140 KT 12HR VT 13/1200Z 22.2N 60.9W 135 KT 24HR VT 14/0000Z 22.8N 63.0W 130 KT 36HR VT 14/1200Z 23.4N 65.1W 130 KT 48HR VT 15/0000Z 24.0N 67.0W 130 KT 72HR VT 16/0000Z 25.0N 70.0W 125 KT 96HR VT 17/0000Z 27.0N 72.5W 115 KT 120HR VT 18/0000Z 29.5N 74.5W 105 KT NNNN
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