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| #211918 (Received by flhurricane at: 4:58 AM 31.Aug.2008) TCDAT3 TROPICAL STORM HANNA DISCUSSION NUMBER 13 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 500 AM EDT SUN AUG 31 2008 HANNA HAS BEEN GOING THROUGH SOME STRUCTURAL CHANGES THAT HAVE RESEMBLED A SUBTROPICAL CYCLONE AT TIMES AS AN UPPER-LEVEL COLD LOW AND DRY AIR HAVE BECOME ENTANGLED WITH THE INNER-CORE CIRCULATION. HOWEVER...DURING THE PAST 6 HOURS WATER VAPOR IMAGERY HAS INDICATED THAT THE UPPER-LOW HAS BEGUN TO DROP SOUTHWARD TOWARD HISPANIOLA ...WHICH MAY ALLOW HANNA TO TRANSITION BACK INTO A FULL WARM CORE TROPICAL CYCLONE LATER TODAY. THE INTENSITY WAS INCREASED TO 50 KT AND THE WIND RADII WERE EXPANDED SIGNIFICANTLY IN THE NORTHERN SEMICIRCLE BASED ON A 30/2231Z QUIKSCAT OVERPASS THAT WAS NOT AVAILABLE FOR THE PREVIOUS ADVISORY. SATELLITE ESTIMATES FROM TAFB AND SAB ARE ST3.0/45 KT. THE INITIAL MOTION ESTIMATE AT 290/10. ALL OF THE NHC MODEL GUIDANCE NOW INDICATES SOME SOUTHWARD COMPONENT TO HANNA'S TRACK BY 48 HOURS AS STRONG NORTHERLY FLOW IMPINGES ON THE CYCLONE BEHIND A DIGGING TROUGH OFF THE U.S. EAST COAST COMBINED WITH OUTFLOW FROM HURRICANE GUSTAV LOCATED WELL TO THE WEST OF HANNA. THE SHIPS MODEL INDICATES AT LEAST 30 KT OF NORTHERLY SHEAR ACROSS HANNA AT 48 HOURS...WHICH SHOULD ACT TO DISPLACE ANY DEEP CONVECTION INTO THE SOUTHERN SEMICIRCLE AND ALSO HELP TO FORCE THE CYCLONE SOUTHWARD. SHOULD HANNA BE STRIPPED OF ALL CONVECTION AROUND THAT TIME PERIOD...THE SHALLOW BAM MODEL ALSO INDICATES A SOUTHWARD COMPONENT OF MOTION ...INCLUDING A CYCLONIC LOOP. HOWEVER...BY 96 HOURS... NEARLY ALL OF THE MODELS FORECAST THE STRONG UPPER-TROUGH TO LIFT OUT TO THE NORTHEAST AND BE REPLACED BY A LARGE RIDGE TO THE NORTH AND EAST OF HANNA. THE LATTER FEATURE IS EXPECTED TO GRADUALLY INDUCE A NORTHWESTWARD COMPONENT OF MOTION BY DAYS 4 AND 5. THE KEY TO THE FORECAST TRACK AT THOSE FORECAST TIMES WILL BE JUST HOW FAR SOUTH AND WEST HANNA MOVES BEFORE IT BEGINS TO MOVE NORTHWESTWARD. MOST OF THE MODEL GUIDANCE KEEPS HANNA OVER OR JUST EAST OF THE BAHAMAS...THE EXCEPTION BEING THE UKMET AND ECMWF MODELS...WHICH BRING HANNA NEAR THE NORTH-CENTRAL COAST OF CUBA BY DAY 4 AND INTO THE FLORIDA STRAITS BY DAY 5. AT THIS TIME...THE SPECIFICS OF THOSE TWO TRACKS ARE CONSIDERED TO BE OUTLIERS...BUT THEY HAVE NOT BEEN ENTIRELY DISCOUNTED. THE OFFICIAL TRACK IS CLOSE TO THE PREVIOUS FORECAST TRACK AND THE NHC MODEL CONSENSUS. THE INTENSITY FORECAST IS SOMEWHAT TRICKY. WITH MORE THAN 30 KT OF SHEAR FORECAST TO AFFECT HANNA BY 48 HOURS...THE CYCLONE COULD EASILY BE SHEARED APART. HOWEVER...WHILE THE SHEAR WILL BE STRONG...ITS NEGATIVE EFFECTS MAY BE OFFSET BY THE STRONG UPPER-LEVEL DIFLUENT PATTERN THAT IS ALSO FORECAST TO EXIST ACROSS HANNA AT THAT TIME. BY 72-96 HOURS...ALL OF THE MODELS AGREE ON ANTICYCLONIC OUTFLOW REPLACING THE STRONG NORTHERLY SHEAR PATTERN...INCLUDING OUTFLOW JETS DEVELOPING TO THE NORTH AND SOUTH...WHICH WOULD FAVOR SIGNIFICANT STRENGTHENING OVER 29-30C SSTS BENEATH THE CYCLONE. HOWEVER...THE INTENSITY FORECAST HAS BEEN HELD BELOW ALL OF THE AVAILABLE MODEL GUIDANCE DUE TO THE UNCERTAINTY OF WHAT THE EXACT STRUCTURE OF HANNA WILL BE AFTER THE UPCOMING STRONG SHEAR PATTERN ABATES IN AROUND 72 HOURS. FORECAST POSITIONS AND MAX WINDS INITIAL 31/0900Z 23.2N 69.0W 50 KT 12HR VT 31/1800Z 23.6N 70.2W 50 KT 24HR VT 01/0600Z 23.7N 71.4W 50 KT 36HR VT 01/1800Z 23.7N 72.5W 50 KT 48HR VT 02/0600Z 23.3N 73.1W 50 KT 72HR VT 03/0600Z 23.6N 73.9W 55 KT 96HR VT 04/0600Z 25.1N 75.4W 60 KT 120HR VT 05/0600Z 28.0N 78.0W 70 KT $$ FORECASTER STEWART |
