Tropical Depression GABRIELLE

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TROPICAL DEPRESSION GABRIELLE DISCUSSION NUMBER  16
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072013
500 AM AST THU SEP 12 2013

A NEW BURST OF CONVECTION HAS FORMED JUST EAST OF THE CENTER OF
GABRIELLE DURING THE PAST FEW HOURS.  THE SATELLITE INTENSITY
ESTIMATE FROM TAFB IS 30 KT...AND AN EARLIER OSCAT OVERPASS SHOWED
25-30 KT WINDS NORTHEAST OF THE CENTER.  BASED ON THESE DATA...THE
INITIAL INTENSITY REMAINS 30 KT.  ALTHOUGH GABRIELLE REMAINS
SHEARED...ANALYSES FROM CIMSS INDICATE THAT THE SHEAR HAS DECREASED
TO 10-15 KT OVER THE PAST 24 HOURS.
 
THE INITIAL MOTION IS NOW 305/6.  WATER VAPOR IMAGERY AND SURFACE
OBSERVATIONS SHOW A DEEP-LAYER TROUGH AND ASSOCIATED SURFACE COLD
FRONT MOVING INTO THE NEW ENGLAND STATES.  THE DYNAMICAL MODELS
FORECAST A LOW TO FORM ON THE FRONT DURING THE NEXT 48 HOURS.  THIS
EVOLUTION SHOULD CREATE A BREAK IN THE SUBTROPICAL RIDGE OVER THE
WESTERN ATLANTIC AND ALLOW GABRIELLE TO RECURVE INTO THE
WESTERLIES.  THE NEW FORECAST TRACK IS SIMILAR TO THE PREVIOUS
TRACK...CALLING FOR A TURN TOWARD THE NORTH DURING THE NEXT 24
HOURS FOLLOWED BY A NORTH-NORTHEASTWARD ACCELERATION TOWARD
ATLANTIC CANADA.  THE TRACK GUIDANCE CONTINUES TO AGREE ON THIS
SCENARIO...AND THE NEW TRACK IS AGAIN AN UPDATE OF THE PREVIOUS
ADVISORY.
 
THE INTENSITY FORECAST IS A LITTLE TRICKY.  ON ONE SIDE...THE
DECREASED SHEAR...24 HOURS BEFORE CROSSING THE NORTH WALL OF THE
GULF STREAM...AND POSSIBLE INTERACTION WITH A BAROCLINIC TROUGH
SUGGEST SOME POTENTIAL FOR STRENGTHENING.  ON THE OTHER HAND...THE
CYCLONE IS LIKELY TO MERGE WITH THE NON-TROPICAL SYSTEM IN 36-48
HOURS...AND THE DYNAMICAL MODELS ARE IN GOOD AGREEMENT IN THE
CYCLONE DECAYING TO A TROUGH IN 36 HOURS OR LESS.  ON THE PREMISE
THAT THE PROS WILL OUTWEIGH THE CONS...THE INTENSITY FORECAST SHOWS
MODEST STRENGTHENING BEFORE EXTRATROPICAL TRANSITION.  THE
ALTERNATIVE SCENARIO REMAINS THAT GABRIELLE COULD DECAY TO A
REMNANT LOW OR TROUGH AT ANY TIME DURING THE NEXT 48 HOURS.  AFTER
EXTRATROPICAL TRANSITION...THE CYCLONE OR ITS REMNANTS WILL QUICKLY
BE ABSORBED BY THE LARGER NON-TROPICAL LOW.
 

FORECAST POSITIONS AND MAX WINDS
 
INIT  12/0900Z 33.5N  67.7W   30 KT  35 MPH
 12H  12/1800Z 34.4N  67.9W   30 KT  35 MPH
 24H  13/0600Z 36.9N  67.5W   35 KT  40 MPH
 36H  13/1800Z 41.2N  65.4W   35 KT  40 MPH
 48H  14/0600Z 46.7N  61.8W   35 KT  40 MPH...POST-TROP/EXTRATROP
 72H  15/0600Z...DISSIPATED
 
$$
FORECASTER BEVEN
 
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