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| #471336 (Received by flhurricane at: 9:41 PM 08.Nov.2011) TCDAT4 TROPICAL STORM SEAN DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL192011 1000 PM EST TUE NOV 08 2011 THE STRUCTURE OF SEAN HAS NOT CHANGED MUCH OVER THE PAST FEW HOURS. THE CLOUD PATTERN CONSISTS OF A PRIMARY BAND OF RAGGED CONVECTION THAT WRAPS MOST OF THE WAY AROUND THE CENTER...WHILE THE COLDEST CLOUD TOPS AND LIGHTNING ACTIVITY ARE SEEN IN A SEPARATE BAND DISPLACED ABOUT 175 NM EAST OF THE CENTER. DVORAK ESTIMATES FROM TAFB AND SAB SUPPORT MAINTAINING THE INITIAL INTENSITY OF 45 KT FOR THIS ADVISORY. WHILE SEAN IS CURRENTLY SITUATED OVER MARGINAL SSTS...THE UPPER LEVELS OF THE ATMOSPHERE ARE COOLER THAN NORMAL... WHICH SHOULD SUSTAIN ENOUGH CONVECTION TO ALLOW FOR SOME STRENGTHENING. THE NHC INTENSITY FORECAST REFLECTS THIS REASONING AND IS UNCHANGED FROM THE PREVIOUS ADVISORY...SHOWING SEAN PEAKING IN AROUND 36 HOURS. THIS FORECAST IS A LITTLE ABOVE THE LGEM AND CLOSE TO THE SPC3 SHIPS ENSEMBLE. AFTER 48 HOURS SEAN WILL MOVE OVER COOLER WATERS AND INTO A HIGHER SHEAR ENVIRONMENT...WHICH SHOULD RESULT IN SOME WEAKENING AS THE CYCLONE BECOMES EXTRATROPICAL BY 72 HOURS. SEAN SHOULD BE ABSORBED BY A SURFACE FRONT SHORTLY AFTER TRANSITION...AND IN FACT SOME GLOBAL MODEL FIELDS SHOW THAT OCCURRING EARLIER THAN INDICATED HERE. SEAN HAS MOVED SLOWLY WESTWARD OVER THE PAST FEW HOURS...WITH AN INITIAL MOTION ESTIMATE OF 270/03. A NARROW MID-LEVEL RIDGE BUILDING NORTH OF THE CYCLONE SHOULD RESULT IN A SLOW WEST- NORTHWESTWARD MOTION DEVELOPING IN THE NEXT 12 HOURS OR SO... FOLLOWED BY A TURN TOWARD THE NORTH IN ABOUT 24 HOURS AS THE RIDGE WEAKENS. SEAN SHOULD THEN BEGIN TO ACCELERATE NORTHEASTWARD BY 48 HOURS AS A DEEP-LAYER TROUGH MOVES OFF THE EAST COAST OF THE UNITED STATES. DUE TO THE MORE WESTERLY INITIAL MOTION THE NHC FORECAST HAS BEEN ADJUSTED A BIT TO THE LEFT OF THE PREVIOUS ONE FOR THE FIRST 12 TO 24 HOURS. BEYOND 24 HOURS...THE LATEST RUN OF THE GFS HAS SHIFTED TO THE LEFT...BUT THE NHC FORECAST HAS ONLY BEEN NUDGED A LITTLE IN THAT DIRECTION AT 48 AND 72 HOURS...AND STILL LIES ON THE EASTERN SIDE OF THE GUIDANCE ENVELOPE. THE INITIAL 34-KT WIND RADII WERE EXPANDED IN THE NORTHERN SEMICIRCLE BASED ON SEVERAL PEAK 1-MINUTE WIND OBSERVATIONS ABOVE TROPICAL STORM FORCE AT NOAA BUOY 41048. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 27.9N 70.1W 45 KT 50 MPH 12H 09/1200Z 28.0N 70.5W 50 KT 60 MPH 24H 10/0000Z 28.6N 70.6W 50 KT 60 MPH 36H 10/1200Z 29.5N 70.1W 55 KT 65 MPH 48H 11/0000Z 31.0N 68.4W 50 KT 60 MPH 72H 12/0000Z 36.5N 61.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 13/0000Z...ABSORBED BY FRONT $$ FORECASTER BRENNAN |
