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Chill falls over the Atlantic as the official season heads towards close
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| #6936 (Received by flhurricane at: 10:49 PM 31.Aug.2004) TCDAT1 HURRICANE FRANCES DISCUSSION NUMBER 29 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 11 PM EDT TUE AUG 31 2004 RECENT RECONNAISSANCE AIRCRAFT REPORTS INDICATE THE CENTRAL PRESSURE HAS STABILIZED TO AROUND 939-940 MB...BASED ON THE 700 MB HEIGHT FALLS. THE MAXIMUM FLIGHT-LEVEL WIND REPORTED ON THE LAST LEG WAS 130 KT ...WHICH EQUATES TO ABOUT A 117-KT SURFACE WIND. A DROPSONDE IN THE NORTHEAST QUADRANT ALSO REPORTED A 135-KT WIND AT ABOUT 600 FT ABOVE THE OCEAN SURFACE. CLOUD TOPS HAVE ALSO COOLED AROUND THE EYE AND THE EYE HAS BECOME MORE DISTINCT IN BOTH SATELLITE AND RADAR DATA...WHILE CONTRACTING DOWN TO ABOUT 20 NMI. BASED ON THIS INFORMATION...THE INTENSITY IS BEING MAINTAINED AT 120 KT. THE INITIAL MOTION IS NOW 285/14. THE FORECAST IS RELATIVELY CLEAR CUT THROUGH 48 HOURS WITH MOST OF THE NHC MODEL GUIDANCE AGREEING ON A WEST-NORTHWESTWARD MOTION TOWARD THE EAST-CENTRAL BAHAMAS. AFTER THAT...THE MODELS DIVERGE AND DIVIDE INTO THREE DISTINCT TRACK PATTERNS. THE GFDL AND GFDN RACE FRANCES TOWARD THE SOUTHERN COAST OF SOUTH CAROLINA IN 96-120 HR...THE GFS...MEDIUM AND DEEP BAM... AND UKMET MODELS TAKE FRANCES MORE SLOWLY INTO EAST-CENTRAL FLORIDA IN 96-120 HR...WHILE THE NOGAPS...LBAR..12Z ECMWF...AND THE ETA MODELS MOVE FRANCES ACROSS SOUTHEAST FLORIDA IN ABOUT 72 HR. THE 18Z GFDL MODEL ALSO SHIFTED WESTWARD FROM THE PREVIOUS RUN. IF THE GFDN AND GFDL MODELS ARE IGNORED...THE REST OF THE NHC MODEL GUIDANCE AGREES THAT THE FLORIDA EAST COAST WILL BE THE EVENTUAL LANDFALL REGION. ONE DISTURBING PIECE OF INFORMATION IS THAT ALL OF THE MODELS...EXCLUDING THE GFDL AND GFDN...AGREE THAT FRANCES WILL SLOW DOWN TO LESS THAN 10 KT AFTER 72 HOURS...AND THE GFS EVEN REDUCES THE FORWARD SPEED TO 4 KT IN 96 HR. THIS SUGGESTS THAT STEERING CURRENTS MAY BRIEFLY COLLAPSE AS FRANCES APPROACHES THE NORTHWESTERN BAHAMAS AND THE FLORIDA EAST COAST. THE DIFFERENCE IN THE MODELS IS HOW THEY HANDLE THE DEEP LOW OFF THE NORTHWESTERN U.S. COAST AS IT DIGS INTO THE CENTRAL PLAINS...AND HOW THE RIDGE AMPLIFIES ALONG THE U.S. EAST COAST IN RESPONSE TO THAT DIGGING LOW/TROUGH. THE GFDL HAS BEEN CONSISTENTLY INITIALIZING TOO WEAK OF A RIDGE AROUND BERMUDA...SO LITTLE WEIGHT WAS PLACED ON THE GFDL/GFDN SOLUTIONS. A SLIGHT WESTWARD JOG WAS PLACED IN THE FORECAST TRACK AFTER 72-96 HR TO INDICATE SOME BUILDING OF THE RIDGE TO THE NORTH OF FRANCES AT THAT TIME. THE OFFICIAL TRACK IS CLOSE TO THE PREVIOUS TRACK THROUGH 72 HOURS...AND THEN A LITTLE LEFT OF TRACK...IN ACCORDANCE WITH THE GUNA CONSENSUS MODEL. THE INTENSITY FORECAST REMAINS UNCHANGED. THE SHIPS MODEL PEAKS FRANCES AT 124 KT IN 12-24 HOURS...THEN WEAKENS THE HURRICANE DOWN TO 112 KT IN 72 HOURS. HOWEVER...THIS WEAKENING IS CAUSED BY 'SELF-INFLICTED' VERTICAL SHEAR SINCE THE OFFICIAL FORECAST TRACK IS SOUTH OF THE GFS TRACK...PLACING THE HURRICANECENTER IN STRONG CYCLONIC WESTERLY OUTFLOW ON THE SOUTH SIDE OF THE GFS MODEL STORM. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 01/0300Z 20.8N 67.0W 120 KT 12HR VT 01/1200Z 21.7N 69.1W 125 KT 24HR VT 02/0000Z 22.8N 71.4W 130 KT 36HR VT 02/1200Z 24.0N 73.5W 130 KT 48HR VT 03/0000Z 25.2N 75.5W 130 KT 72HR VT 04/0000Z 26.8N 78.3W 125 KT 96HR VT 05/0000Z 28.0N 81.0W 100 KT...INLAND 120HR VT 06/0000Z 31.0N 82.5W 55 KT...INLAND |
