[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 080927
TCDAT4

SUBTROPICAL STORM SEAN DISCUSSION NUMBER   1...CORRECTED
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL192011
400 AM EST TUE NOV 08 2011

CORRECTED TIME FROM EDT TO EST

SATELLITE INFORMATION AND SURFACE OBSERVATIONS INDICATE THE LARGE
NON-TROPICAL LOW PRESSURE SYSTEM THAT HAS BEEN MEANDERING BETWEEN
BERMUDA AND THE BAHAMAS THE PAST FEW DAYS HAS ACQUIRED ENOUGH
ORGANIZED CONVECTION TO BE CLASSIFIED AS SUBTROPICAL STORM SEAN. AN
08/1021Z ASCAT OVERPASS INDICATED THE ENTIRE LOW-LEVEL CIRCULATION
HAD BECOME BETTER DEFINED...ESPECIALLY THE INNER CORE WIND FIELD. A
LARGE FIELD OF 34-KT WINDS EXTENDS A FEW HUNDRED MILES WEST THROUGH
NORTH OF THE CENTER...BUT THE ASCAT OVERPASS APPEARS TO HAVE
UNDERESTIMATED THE ACTUAL STRENGTH OF WINDS BASED ON WINDS OF 41 KT
REPORTED BY NOAA BUOYS 41047 AND 41048 AT 08/0100Z. SINCE THE TIME
OF THE ASCAT PASS...THUNDERSTORM ACTIVITY HAS INCREASED IN ALL
QUADRANTS...AND HAS BECOME MORE CONCENTRATED NEAR THE LOW-LEVEL
CENTER. FURTHERMORE...WEAK ANTICYCLONIC OUTFLOW HAS STARTED TO
DEVELOP DURING THE PAST FEW HOURS...AND THE 08/00Z FSU PHASE
EVOLUTION DIAGRAMS BASED OFF THE GFS...UKMET...AND CANADIAN MODELS
INDICATE THIS SYSTEM HAS DEVELOPED A SHALLOW-MODERATE DEPTH WARM
CORE. ALL OF THESE DATA GIVE CREDENCE THAT THIS SYSTEM IS AT LEAST
A SUBTROPICAL CYCLONE. THE INITIAL INTENSITY OF 40 KT IS BASED ON
THE NOAA BUOY REPORTS AND SATELLITE SUBTROPICAL CLASSIFICATIONS OF
ST2.5/35-40 KT FROM BOTH TAFB AND SAB.

THE INITIAL MOTION IS NEARLY STATIONARY. SEAN HAS BEEN MAKING A
SMALL CYCLONIC LOOP DURING THE PAST SEVERAL HOURS AND LITTLE
OVERALL MOTION IS EXPECTED TODAY AS THE CYCLONE REMAINS TRAPPED IN
WEAK STEERING FLOW. BY 12-24 HOURS...HOWEVER...SEAN IS EXPECTED TO
MOVE SLOWLY NORTHWESTWARD AS THE SUBTROPICAL RIDGE TO THE NORTH
BEGINS TO ERODE AND RETREAT TO THE EAST AS A STRONG FRONTAL SYSTEM
AND ASSOCIATED DEEP-LAYER TROUGH MOVE EASTWARD ACROSS THE EASTERN
UNITED STATES. BY 48-60 HOURS...THE AFOREMENTIONED FRONTAL SYSTEM
WILL BE MOVING OFF THE U.S. EAST COAST...AND THE STRONG
SOUTHWESTERLY FLOW ASSOCIATED WITH THE FRONT SHOULD BEGIN TO
ACCELERATE SEAN TOWARD THE NORTHEAST. IN THE 96-120 TIME FRAME...
THE CYCLONE IS EXPECTED TO MERGE WITH THE FRONTAL SYSTEM...ALTHOUGH
DISSIPATION OF SEAN DUE TO STRONG VERTICAL SHEAR IS A VERY DISTINCT
POSSIBILITY. THE OFFICIAL FORECAST TRACK IS TO THE RIGHT OF THE
CONSENSUS MODELS...AND IS A BLEND OF THE GFS AND ECMWF MODELS.

SEAN IS EXPECTED TO REMAIN OVER 26C SSTS FOR AT LEAST THE NEXT 48
HOURS...WHICH MEANS THE CYCLONE COULD POSSIBLY TRANSITION INTO A
TROPICAL STORM AS THE VERTICAL SHEAR WEAKENS. HOWEVER...ALL THIS
WOULD LIKELY DO IS CONTRACT THE WIND FIELD. OVERALL...ONLY A SLIGHT
INCREASE IN STRENGTH IS EXPECTED...ASSUMING THAT AN EYE FEATURE
DOES NOT DEVELOP. I SHOULD BE NOTED THAT SEAN IS A RELATIVELY
SHALLOW SYSTEM WHOSE VERTICAL EXTENT LIKELY ONLY EXTENDS UP TO THE
300 MB LEVEL. AS A RESULT...THE NORMAL SHEAR COMPUTATIONS MADE BY
THE SHIPS MODEL ARE LIKELY GOING TO BE TOO STRONG SINCE THAT MODELS
USES WINDS AT THE 200 MB LEVEL.

FORECAST POSITIONS AND MAX WINDS

INIT  08/0900Z 27.2N  69.4W   40 KT  45 MPH
 12H  08/1800Z 27.2N  69.7W   40 KT  45 MPH
 24H  09/0600Z 27.4N  70.0W   45 KT  50 MPH
 36H  09/1800Z 28.2N  70.5W   45 KT  50 MPH
 48H  10/0600Z 29.2N  70.8W   45 KT  50 MPH
 72H  11/0600Z 31.5N  68.8W   45 KT  50 MPH
 96H  12/0600Z 37.5N  59.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  13/0600Z...DISSIPATED

$$
FORECASTER STEWART



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