Tropical Storm SEAN

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TROPICAL STORM SEAN DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL192011
500 PM AST TUE NOV 08 2011
 
CONVECTION HAS FORMED NEAR THE CENTER OF SEAN THIS AFTERNOON WITH A
BANDING FEATURE PERSISTING IN THE NORTHEASTERN QUADRANT.  SINCE THE
STORM IS A LITTLE FARTHER FROM THE UPPER-LEVEL LOW...WITH A WARM
CORE APPARENT ON AMSU ANALYSES AND A SMALLER RADIUS OF MAXIMUM
WINDS AS SEEN IN ASCAT DATA...SEAN NOW HAS ENOUGH CHARACTERISTICS
TO BE CONSIDERED TROPICAL.  THE MAXIMUM WINDS ARE SET TO 45 KT...IN
AGREEMENT WITH THE TAFB DVORAK ESTIMATE AND EARLIER ASCAT WINDS. 
WHILE THE REGIONAL HURRICANE MODELS DO NOT SHOW MUCH
INTENSIFICATION OF SEAN...THE SHEAR REMAINS RELATIVELY LIGHT FOR
THE NEXT 36 HR OR SO WHILE THE SYSTEM IS OVER MARGINAL SSTS.  SINCE
THE STORM IS EMBEDDED WITHIN COLDER-THAN-AVERAGE UPPER-AIR
TEMPERATURES...THE SST THRESHOLD FOR TROPICAL CYCLONE STRENGTHENING
IS LOWER THAN USUAL...AND SOME FURTHER STRENGTHENING IS
ANTICIPATED.  THE NEW NHC FORECAST IS HIGHER THAN THE PREVIOUS
ONE...AND REMAINS IN LINE WITH THE SHIPS/LGEM MODELS AND THE
EXPERIMENTAL SPC3 ENSEMBLE SHIPS GUIDANCE.  IN ABOUT TWO DAYS...
SEAN SHOULD ENCOUNTER A MORE HOSTILE UPPER-AIR ENVIRONMENT...WHICH
SHOULD INDUCE WEAKENING.  
 
THE CENTER OF THE CYCLONE HAS BEEN WOBBLING AROUND FOR QUITE SOME
TIME...BUT IT APPEARS THAT A MOTION TOWARD THE WEST-NORTHWEST HAS
BEGUN.  THIS GENERAL TRACK IS EXPECTED TO CONTINUE THROUGH TOMORROW
AS A RIDGE MOVES THROUGH THE WESTERN ATLANTIC.  THEREAFTER...THE
STORM SHOULD TURN TOWARD THE NORTH AND NORTHEAST BY THURSDAY AHEAD
OF A DEEP TROUGH OVER THE EASTERN UNITED STATES.  IT SEEMS THAT
BEING ON THE RIGHT SIDE OF THE GUIDANCE EARLIER WAS A GOOD CALL...
AS MOST OF THE MODELS HAVE SHIFTED IN THAT DIRECTION...APPARENTLY
BECAUSE OF FLATTER MID-LATITUDE FLOW FORECAST TO AFFECT SEAN.  ONLY
COSMETIC CHANGES WERE MADE TO THE NHC TRACK FOR THE FIRST COUPLE OF
DAYS...WITH A SHIFT TO THE SOUTHEAST MADE AT DAY 3...BETWEEN THE
ECMWF AND GFS MODELS.  ALL OF THE GLOBAL MODELS SHOW THE STORM
BEING ABSORBED BY A FRONTAL BOUNDARY BY DAY 4...AND THIS SCENARIO
IS REFLECTED IN THE NHC FORECAST.  
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  08/2100Z 27.8N  69.8W   45 KT  50 MPH
 12H  09/0600Z 27.9N  70.2W   50 KT  60 MPH
 24H  09/1800Z 28.4N  70.4W   50 KT  60 MPH
 36H  10/0600Z 29.0N  70.5W   55 KT  65 MPH
 48H  10/1800Z 29.9N  69.7W   50 KT  60 MPH
 72H  11/1800Z 34.0N  64.0W   45 KT  50 MPH
 96H  12/1800Z...ABSORBED
 
$$
FORECASTER BLAKE
 
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