[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 090240
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



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