[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 100836
TCDAT4

TROPICAL STORM SEAN DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL192011
400 AM EST THU NOV 10 2011

INFRARED AND MICROWAVE SATELLITE IMAGERY THIS MORNING SHOWS THAT
SEAN IS SLOWLY BECOMING BETTER ORGANIZED...WITH INCREASING
CONVECTIVE BANDING AROUND THE CENTER AND OCCASIONAL ATTEMPTS AT EYE
FORMATION.  SATELLITE INTENSITY ESTIMATES FROM SAB AND TAFB ARE 55
KT...AND THAT REMAINS THE INITIAL INTENSITY.  THE CYCLONE CURRENTLY
HAS GOOD CIRRUS OUTFLOW IN THE NORTH SEMICIRCLE.

SEAN CONTINUES ITS NORTHERLY TRACK WITH AN INITIAL MOTION OF 355/7.
WATER VAPOR IMAGERY SHOWS SOUTHWESTERLY MID/UPPER-LEVEL FLOW
ASSOCIATED WITH A TROUGH OVER THE EASTERN UNITED STATES APPROACHING
THE CYCLONE...AND THE GUIDANCE IS IN GOOD AGREEMENT THAT THIS
SHOULD CAUSE SEAN TO TURN NORTHEASTWARD IN THE NEXT 12 HOURS.
AFTER THAT...THE CYCLONE SHOULD ACCELERATE NORTHEASTWARD DURING THE
REMAINDER OF ITS LIFETIME.  THE NEW FORECAST TRACK FOLLOWS THIS
SCENARIO AND IS SIMILAR TO...BUT NORTH OF...THE PREVIOUS TRACK.
THE FORECAST TRACK IS NEAR THE CENTER OF THE TRACK GUIDANCE
ENVELOPE.

THERE IS PERHAPS 12-24 HOURS FOR SEAN TO STRENGTHEN BEFORE STRONG
VERTICAL WIND SHEAR IMPACTS THE STORM.  GIVEN THE CURRENT
STRUCTURE...THE INTENSITY FORECAST CALLS FOR SEAN TO REACH HURRICANE
STRENGTH IN 24 HOURS IN AGREEMENT WITH THE PREVIOUS FORECAST.
AFTER THAT...STRONG SHEAR AND THE START OF EXTRATROPICAL TRANSITION
SHOULD CAUSE WEAKENING.  EXTRATROPICAL TRANSITION IS EXPECTED TO BE
COMPLETE BY 48 HOURS...AND BY 72 HOURS SEAN IS EXPECTED TO BE
ABSORBED BY A LARGER BAROCLINIC SYSTEM TO ITS NORTH.  AS NOTED IN
THE PREVIOUS DISCUSSION...IT IS POSSIBLE THAT SEAN COULD DISSIPATE
EARLIER THAN CURRENTLY FORECAST.

FORECAST POSITIONS AND MAX WINDS

INIT  10/0900Z 30.2N  70.9W   55 KT  65 MPH
 12H  10/1800Z 31.2N  69.8W   60 KT  70 MPH
 24H  11/0600Z 33.4N  67.2W   65 KT  75 MPH
 36H  11/1800Z 37.0N  63.0W   55 KT  65 MPH
 48H  12/0600Z 41.5N  56.5W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  13/0600Z...ABSORBED BY EXTRATROPICAL LOW

$$
FORECASTER BEVEN


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