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Tropical Storm GABRIELLE (Text)

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TROPICAL STORM GABRIELLE DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072013
500 AM AST WED SEP 11 2013

SATELLITE IMAGERY INDICATES THAT GABRIELLE IS COMPRISED OF A SWIRL
OF LOW/MID-LEVEL CLOUDS WITH NO DEEP CONVECTION NEAR THE CENTER AT
THIS TIME.  THIS IS LIKELY DUE TO THE ONGOING AFFECT OF 15-20 KT OF
SOUTHWESTERLY VERTICAL WIND SHEAR.  AN EARLIER ASCAT OVERPASS...
RECENT REPORTS FROM AN AIR FORCE RESERVE HURRICANE HUNTER
AIRCRAFT...AND OBSERVATIONS FROM BERMUDA SHOW THAT THE WINDS HAVE
DECREASED DURING THE PAST 12 HOURS.  THE INITIAL INTENSITY IS
REDUCED TO 45 KT...AND THIS COULD BE GENEROUS.

THE CYCLONE HAS TURNED TO THE LEFT SINCE THE PREVIOUS ADVISORY...AND
THE INITIAL MOTION IS NOW 325/7.  THIS IS PARTLY DUE TO AN UPPER-
LEVEL LOW TO THE SOUTHWEST OF GABRIELLE AND A STRENGTHENING
LOW/MID-LEVEL RIDGE TO THE NORTH.  THIS COMBINATION SHOULD STEER
GABRIELLE SLOWLY TOWARD THE NORTHWEST FOR THE NEXT 24 HOURS OR SO. 
AFTER THAT...THE CYCLONE IS LIKELY TO RECURVE ON THE EASTERN SIDE
OF A LARGE DEEP-LAYER TROUGH MOVING INTO THE EASTERN UNITED STATES
AND THE WESTERN ATLANTIC.  WHILE THE TRACK MODEL GUIDANCE IS IN
GOOD AGREEMENT WITH THIS SCENARIO...IT HAS SHIFTED QUITE A BIT TO
THE LEFT/WEST DURING THE FIRST 48 HOURS.  THE NEW FORECAST TRACK IS
ALSO SHIFTED TO THE LEFT DURING THAT TIME...BUT STILL LIES TO THE
RIGHT OF THE CONSENSUS MODELS AND THE MIDDLE OF THE GUIDANCE
ENVELOPE.

GABRIELLE IS FORECAST TO REMAIN OVER WARM SEA SURACE TEMPERATURES
FOR THE NEXT 48 HOURS.  HOWEVER...THE DYNAMICAL MODELS CALL FOR
CONTINUED STRONG SHEAR...WITH THE UKMET...NAVGEM...AND ECMWF MODELS
FORECASTING THE CYCLONE TO DECAY TO A TROUGH DURING THIS TIME. 
AFTER THAT...WHATEVER IS LEFT OF THE CYCLONE IS EXPECTED TO MERGE
WITH A FRONTAL SYSTEM BETWEEN 48-72 HOURS.  THE INTENSITY FORECAST
FOLLOWS THE GFS SCENARIO THAT GABRIELLE WILL REMAIN A TROPICAL
CYCLONE UNTIL IT MERGES WITH THE FRONT...WITH THE NEW FORECAST A
LITTLE WEAKER THAN THE PREVIOUS FORECAST.  HOWEVER...AN ALTERNATIVE
SCENARIO IS THAT THE CYCLONE DECAYS TO A REMNANT LOW OR TROUGH
DURING THE NEXT 48 HOURS DUE TO A LACK OF CONVECTION.

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  11/0900Z 32.5N  65.7W   45 KT  50 MPH
 12H  11/1800Z 33.1N  66.2W   45 KT  50 MPH
 24H  12/0600Z 33.7N  66.8W   45 KT  50 MPH
 36H  12/1800Z 34.7N  67.3W   40 KT  45 MPH
 48H  13/0600Z 37.2N  66.5W   40 KT  45 MPH
 72H  14/0600Z 46.5N  59.5W   40 KT  45 MPH...POST-TROP/EXTRATROP
 96H  15/0600Z...DISSIPATED
 
$$
FORECASTER BEVEN
 
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