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Chill falls over the Atlantic as the official season heads towards close
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| #213533 (Received by flhurricane at: 10:54 AM 04.Sep.2008) TCDAT3 TROPICAL STORM HANNA DISCUSSION NUMBER 31 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 1100 AM EDT THU SEP 04 2008 VERY DRY AIR ASSOCIATED WITH THE UPPER-LEVEL LOW OVER THE NORTHWESTERN BAHAMAS IS CHOKING OFF CONVECTION NEAR THE CORE OF HANNA...AND THE CYCLONE HAS A VERY SUBTROPICAL APPEARANCE. CONVECTION HAS DIMINISHED SUBSTANTIALLY OVER THE PAST 6-12 HOURS AND THE HIGHEST FLIGHT-LEVEL AND SFMR WINDS HAVE COME DOWN AS WELL. BASED ON SOME 52 KT SFMR WINDS AROUND 11Z...THE INITIAL INTENSITY IS LOWERED TO 55 KT. CONDITIONS ARE NOT CONDUCIVE TO STRENGTHENING RIGHT NOW...WITH MODERATE SOUTHERLY SHEAR AND POOR THERMODYNAMICS. THE GFS SUGGESTS THAT DRY AIR IS LIKELY TO CONTINUE TO BE AN ISSUE UP TO LANDFALL. HOWEVER...AS HANNA AND THE UPPER LOW BEGIN TO SEPARATE SOME OF THE GLOBAL MODELS...INCLUDING THE GFS...SUGGEST THAT THE SHEAR WILL LESSEN SOMEWHAT. THIS WOULD ALLOW FOR SOME INTENSIFICATION...AND SO THE OFFICIAL FORECAST CONTINUES TO SHOW HANNA ATTAINING HURRICANE STRENGTH. THIS IS IN GOOD AGREEMENT WITH THE HWRF...ALTHOUGH THE GFDL AND SHIPS GUIDANCE IS LOWER. THE INITIAL MOTION IS 315/10. THE FORECAST SCENARIO IS UNCHANGED FROM PREVIOUS ADVISORIES. MID-LEVEL HIGH PRESSURE IS BUILDING OVER THE WESTERN ATLANTIC AS A DEEP LAYER LOW MOVES AWAY FROM NEW ENGLAND. HANNA IS EXPECTED TO RECURVE AROUND THE PERIPHERY OF THE HIGH AHEAD OF A MID-LATITUDE SHORT WAVE TROUGH IN THE WESTERLIES. MODEL GUIDANCE REMAINS IN FAIRLY GOOD AGREEMENT AND LITTLE CHANGE TO THE FORECAST TRACK IS REQUIRED. THE OFFICIAL FORECAST IS SLIGHTLY TO THE RIGHT OF THE MODEL CONSENSUS BECAUSE THE BETTER PERFORMING MODELS THIS YEAR...NAMELY THE GFS AND GFDL...ARE ON THE RIGHT SIDE OF THE GUIDANCE ENVELOPE. BECAUSE OF THE LARGE... SPRAWLING NATURE OF THE CIRCULATION...THE EXACT TRAJECTORY OF THE CENTER IS RELATIVELY UNIMPORTANT. FORECAST POSITIONS AND MAX WINDS INITIAL 04/1500Z 24.5N 73.5W 55 KT 12HR VT 05/0000Z 26.2N 75.2W 55 KT 24HR VT 05/1200Z 28.7N 77.2W 60 KT 36HR VT 06/0000Z 31.7N 78.2W 65 KT 48HR VT 06/1200Z 35.4N 77.4W 55 KT 72HR VT 07/1200Z 43.0N 69.5W 40 KT...INLAND 96HR VT 08/1200Z 50.0N 54.0W 40 KT...EXTRATROPICAL 120HR VT 09/1200Z 51.5N 37.5W 45 KT...EXTRATROPICAL $$ FORECASTER FRANKLIN |
