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Chill falls over the Atlantic as the official season heads towards close
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| #211350 (Received by flhurricane at: 11:11 PM 28.Aug.2008) TCDAT3 TROPICAL STORM HANNA DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 1100 PM EDT THU AUG 28 2008 AFTER WESTERLY MID- TO UPPER-LEVEL SHEAR PRODUCED AN EXPOSED LOW-LEVEL CIRCULATION CENTER...CONVECTION HAS REDEVELOPED WITH A VENGEANCE OVER OR NEAR THE CENTER CONTAINING CLOUD TOPS AS COLD AS -83C. HOWEVER...THE CONVECTIVE DEVELOPMENT APPEARS TO BE A BURSTING PATTERN...WHICH IS USUALLY SHORT-LIVED. THE ADVISORY INTENSITY IS 45 KT BASED ON A DVORAK T-NUMBER OF T3.0/45 KT FROM TAFB AND THE CONTINUED CONVECTIVE DEVELOPMENT SINCE THE 00Z SATELLITE FIX TIME. THE INITIAL MOTION ESTIMATE IS 290/06. HANNA HAD BEEN MOVING DUE WEST AT ABOUT 10 KT...BUT THE 24-HOUR MOTION OF THE PRESSURE ENVELOPE SURROUNDING THE CYCLONE CENTER HAS ONLY BEEN MOVING AT AROUND 5 KT. THE PRIMARY STEERING MECHANISMS FOR THE NEXT 3 DAYS WILL BE THE LARGE SUBTROPICAL HIGH TO THE NORTH OF HANNA AND THE MID- TO UPPER-LEVEL LOW LOCATED ABOUT 275 NMI WEST OF HANNA. ALL OF THE MODELS HAVE BEEN WEAKENING AND MOVING THE UPPER-LOW TO THE SOUTHWEST TOO QUICKLY...WHICH IS LIKELY DUE TO THE FAST OUTFLOW FROM GUSTAV GETTING ENTRAINED INTO THE LOW AND KEEPING IT STRONGER THAN THE MODEL FORECASTS. HOWEVER...OUTFLOW FROM GUSTAV IS NOW BEING SHUNTED SOUTHEASTWARD INTO THE EASTERN CARIBBEAN...SO A WEAKENING TREND AND SOUTHWESTWARD MOTION BY THE UPPER-LOW IS EXPECTED TO ENSUE WITHIN THE NEXT 24 HOURS. HANNA IS FORECAST TO MOVE AROUND THE UPPER-LOW THROUGH 72-96 HOURS BEFORE IT ENCOUNTERS STRONG MID- TO UPPER-LEVEL NORTHERLY WINDS BEHIND A POWERFUL MID-TROPOSPHERIC TROUGH...WHICH IS FORECAST TO MOVE OFF THE U.S. EAST COAST AT DAYS 4 AND 5. THAT DEVELOPING SCENARIO IS INDICATED BY ALMOST ALL OF THE MODELS TO VARYING DEGREES...BUT ENOUGH TO INDUCE SOME SOUTHWARD COMPONENT TO HANNA'S MOTION. MOST OF THE MODEL GUIDANCE HAS HAD A FAST AND RIGHT-OF-TRACK BIAS THE PAST 36 HOURS...SO THE FORECAST TRACK FOR THIS ADVISORY WAS ADJUSTED TO THE LEFT OF THE PREVIOUS TRACK AND THE NHC MODEL CONSENSUS. MODERATE WESTERLY SHEAR HAS BEEN REPLACED BY LIGHT SOUTHERLY SHEAR DURING THE PAST 6 HOURS NOW THAT THE UPPER-LOW HAS SHOWN SIGNS OF WEAKENING AND MOVING FARTHER AWAY FROM HANNA. AS THE LOW CONTINUES TO WEAKEN AND MOVES TO A POSITION TO THE SOUTHWEST OF HANNA... UPPER-LEVEL OUTFLOW SHOULD IMPROVE TO THE NORTHWEST AIDING A STEADY INTENSIFICATION. ALTHOUGH NOT INDICATED AT ANY OF THE FORECAST TIMES... ATMOSPHERIC AND OCEANIC CONDITIONS ARE EXPECTED TO BECOME QUITE FAVORABLE FOR SIGNIFICANT STRENGTHENING BETWEEN 72 AND 96 HOURS...AND HANNA COULD BECOME A MAJOR HURRICANE AS INDICATED BY THE GFDL MODEL. THE OFFICIAL FORECAST IS A BLEND OF THE SHIPS AND GFDL MODELS...AND IS SIMILAR TO THE LGEM MODEL. FORECAST POSITIONS AND MAX WINDS INITIAL 29/0300Z 21.1N 60.6W 45 KT 12HR VT 29/1200Z 21.8N 62.0W 50 KT 24HR VT 30/0000Z 23.2N 64.2W 55 KT 36HR VT 30/1200Z 24.6N 66.3W 60 KT 48HR VT 31/0000Z 25.7N 68.0W 70 KT 72HR VT 01/0000Z 26.6N 70.0W 80 KT 96HR VT 02/0000Z 26.5N 71.2W 80 KT 120HR VT 03/0000Z 26.0N 71.6W 75 KT $$ FORECASTER STEWART/BEVEN |
