Tropical Storm SEAN

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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
 
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