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| #471391 (Received by flhurricane at: 3:38 AM 09.Nov.2011) TCDAT4 TROPICAL STORM SEAN DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL192011 400 AM EST WED NOV 09 2011 SEAN HAS AGAIN CHANGED LITTLE IN STRUCTURE DURING THE PAST SEVERAL HOURS. A CENTRAL CONVECTIVE BAND IS PRESENT WITHIN 60-75 N MI OF THE CENTER...MAINLY IN THE SOUTHEASTERN SEMICIRCLE...WHILE OUTER CONVECTIVE BANDS ARE PRESENT UP TO 240 N MI FROM THE CENTER IN THE NORTHEASTERN SEMICIRCLE. SATELLITE INTENSITY ESTIMATES FROM SAB AND TAFB ARE 45 KT...AND THAT REMAINS THE INITIAL INTENSITY FOR THIS ADVISORY. THE CENTRAL PRESSURE HAS BEEN ADJUSTED DOWNWARD BASED ON OBSERVATIONS FROM NOAA BUOY 41047. THE INITIAL MOTION REMAINS 270/3. SEAN IS EMBEDDED IN A DEEP-LAYER CYCLONIC CIRCULATION THAT IS CUT OFF FROM THE WESTERLIES SOUTH OF A RIDGE OVER THE NORTHWESTERN ATLANTIC. A STRONG MID/UPPER-LEVEL TROUGH AND ASSOCIATED SURFACE COLD FRONT ARE MOVING EASTWARD THROUGH THE MISSISSIPPI VALLEY...AND THE DYNAMICAL MODELS FORECAST THIS SYSTEM TO REACH THE WESTERN ATLANTIC IN ABOUT 36 HOURS. THIS EVOLUTION SHOULD CAUSE SEAN TO TURN NORTHWARD DURING THE NEXT 24 HOURS...AND THEN TURN NORTHEASTWARD IN DEEP-LAYER SOUTHWESTERLY FLOW AHEAD OF THE APPROACHING TROUGH. WHILE THE DYNAMICAL MODELS AGREE ON THIS SCENARIO...THERE IS SOME SPREAD IN BOTH THE DIRECTION AND SPEED OF FORWARD MOTION AFTER 24 HOURS. THE ECMWF MODEL IS ON THE RIGHT SIDE OF THE GUIDANCE ENVELOPE AND TAKES THE CENTER NEAR BERMUDA...WHILE THE GFDL AND UKMET ARE ON THE LEFT SIDE FORECASTING A TRACK CLOSER TO NOVA SCOTIA AND NEWFOUNDLAND. THE GUIDANCE ENVELOPE HAS SHIFTED TO THE LEFT/NORTH COMPARED TO 6 HOURS AGO... AND THE NEW FORECAST TRACK IS ALSO SHIFTED IN THAT DIRECTION. HOWEVER...OUT OF RESPECT FOR THE ECMWF AND THE PREVIOUS FORECAST THE NEW FORECAST TRACK LIES TO THE RIGHT OF THE CENTER OF THE GUIDANCE ENVELOPE AND THE CONSENSUS MODELS. SEAN IS IN A REGION OF LIGHT/MODERATE VERTICAL WIND SHEAR NEAR THE CENTER OF THE CUT-OFF LOW. THE DYNAMICAL MODELS SUGGEST THE PRESENT SHEAR SHOULD CONTINUE FOR THE NEXT 12-24 HOURS...FOLLOWED BY A SIGNIFICANT INCREASE IN SHEAR CAUSED BY THE TROUGH APPROACHING FROM THE WEST. THIS SHOULD ALLOW SEAN TO STRENGTHEN SOME TODAY... FOLLOWED BY A WEAKENING TREND AFTER 24 HOURS. SEAN IS FORECAST TO BECOME EXTRATROPICAL AS IT MERGES WITH THE APPROACHING COLD FRONT BETWEEN 48-72 HOURS...WITH THE CIRCULATION BECOMING TOTALLY ABSORBED BY THE FRONT BY 96 HOURS. THE NEW INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS FORECAST...WITH JUST A LITTLE LESS STRENGTHENING DURING THE FIRST 24-36 HOURS. IT SHOULD BE NOTED THAT MANY OF THE DYNAMICAL MODELS SHOW THE CIRCULATION OF SEAN DISSIPATING BEFORE 72 HOURS...SO THE CURRENT FORECAST LIFETIME COULD BE GENEROUS. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 27.9N 70.4W 45 KT 50 MPH 12H 09/1800Z 28.4N 70.7W 50 KT 60 MPH 24H 10/0600Z 29.4N 70.7W 50 KT 60 MPH 36H 10/1800Z 30.6N 69.7W 50 KT 60 MPH 48H 11/0600Z 33.1N 67.4W 45 KT 50 MPH 72H 12/0600Z 38.5N 59.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 13/0600Z...DISSIPATED $$ FORECASTER BEVEN |
