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Tropical Storm GABRIELLE Forecast Discussion Number 12
2013-09-11 10:51:16| National Hurricane Center (Atlantic)
Issued at 500 AM AST WED SEP 11 2013 000 WTNT42 KNHC 110851 TCDAT2 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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