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| #8097 (Received by flhurricane at: 4:51 AM 13.Sep.2004) TCDAT4 HURRICANE IVAN DISCUSSION NUMBER 44 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 AM EDT MON SEP 13 2004 A GPS DROPSONDE IN THE NORTHEAST QUADRANT OF THE EYEWALL AT 0510Z MEASURED AN AVERAGE WIND SPEED OF 176 KT THROUGH THE LOWEST 500 METERS OF THE DROP. THIS CONVERTS TO A SURFACE WIND OF 141 KT. SO THE INITIAL WIND SPEED IS KEPT AT 140 KT. THE SHIPS MODEL SHOWS CLOSE TO 20 KT OF WESTERLY SHEAR AFFECTING IVAN BUT RELAXES THIS SHEAR A BIT AT 24 AND 36 HOURS BEFORE INCREASING IT SIGNIFICANTLY. THE OFFICIAL INTENSITY FORECAST SHOWS GRADUAL WEAKENING AFTER 12 HOURS BUT IVAN COULD STILL BE A FORMIDABLE HURRICANE WHEN IT REACHES THE NORTHERN GULF OF MEXICO COAST. THE TRACK SCENARIO REMAINS THE SAME. THE HURRICANE IS FORECAST TO GRADUALLY TURN NORTHWARD OVER THE NEXT 72 HOURS INTO A WEAKNESS IN THE SUBTROPICAL RIDGE...AND THEN NORTHEASTWARD AS IT APPROACHES THE WESTERLIES. THE RIDGE AND ITS WEAKNESS ARE NOT VERY PRONOUNCED AND THE FORWARD MOTION IS EXPECTED TO BE SLOW THROUGHOUT THE FORECAST PERIOD...10 KT OR LESS. WHEN THE FORWARD MOTION IS SLOW ...THE DIRECTION OF MOTION IS LESS CERTAIN. THIS IS SEEN IN THE SPREAD OF THE GLOBAL MODELS. THE GFS SHOWS A LANDFALL IN THE FLORIDA PANHANDLE WHILE THE UKMET AND NOGAPS SHOW A LANDFALL IN SOUTHEASTERN LOUISIANA...ALL IN ABOUT 72 HOURS. THE CONSENSUS OF THE MODELS IS NEAR THE MISSISSIPPI/ALABAMA BORDER. THE INITIAL MOTION ESTIMATE IS 300/08. MOST OF THE GLOBAL TRACK GUIDANCE MODELS HAVE SHIFTED TO THE LEFT FROM THE PREVIOUS MODEL RUNS... EXCEPT THE GFDL WHICH HAS SHIFTED A LITTLE TO THE RIGHT. THE OFFICIAL FORECAST TRACK IS SHIFTED ABOUT 60 N MI LEFT OF THE PREVIOUS ADVISORY AT 72 HOURS AND IS STILL TO THE RIGHT OF ALL GUIDANCE EXCEPT FOR THE GFS. IF I DID NOT HAVE A PREVIOUS FORECAST TO MAINTAIN SOME CONTINUITY WITH...I WOULD HAVE SHIFTED THE TRACK EVEN FURTHER TO THE LEFT. MEANWHILE...SINCE THE MOTION IS STILL WEST-NORTHWESTWARD...THE CENTER COULD MISS THE WESTERN TIP OF CUBA AND MOVE THROUGH THE YUCATAN CHANNEL AND COULD EVEN APPROACH THE YUCATAN PENINSULA IF A 300 DEGREE HEADING IS MAINTAINED FOR ANOTHER 24 HOURS. FORECASTER LAWRENCE FORECAST POSITIONS AND MAX WINDS INITIAL 13/0900Z 20.2N 83.9W 140 KT 12HR VT 13/1800Z 21.2N 84.8W 140 KT 24HR VT 14/0600Z 22.5N 86.0W 135 KT 36HR VT 14/1800Z 24.2N 87.1W 130 KT 48HR VT 15/0600Z 26.0N 87.5W 120 KT 72HR VT 16/0600Z 30.0N 87.5W 105 KT 96HR VT 17/0600Z 33.5N 85.5W 45 KT...INLAND 120HR VT 18/0600Z 36.0N 83.0W 25 KT...INLAND |
