Tropical Storm Gabrielle - 10pm CDT Forecast Discussion

By: National Hurricane Center
By: National Hurricane Center

TROPICAL STORM GABRIELLE DISCUSSION NUMBER 2
NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013
1100 PM AST WED SEP 04 2013

THE SATELLITE PRESENTATION OF THE CYCLONE CONSISTS OF A SMALL AREA OF VERY DEEP CONVECTION NEAR AND NORTHEAST OF THE SURFACE CENTER WHILE THE CONVECTIVE BANDS HAVE LARGELY DISSIPATED OVER THE PAST FEW HOURS. THE LARGE-SCALE OUTFLOW PATTERN OVER THE CYCLONE IS EXCELLENT WITH AN ENHANCED POLEWARD OUTFLOW CHANNEL SEEN IN WATER VAPOR IMAGERY. BASED ON THE LATEST SATELLITE CLASSIFICATIONS FROM BOTH TAFB AND SAB...THE INITIAL INTENSITY IS SET TO 35 KT...AND THE DEPRESSION IS NOW TROPICAL STORM GABRIELLE.

DROPSONDE DATA FROM THE NASA GLOBAL HAWK AIRCRAFT SUGGEST THAT THE CIRCULATION OF GABRIELLE IS TILTED TO THE NORTHEAST WITH HEIGHT... WITH A MID-LEVEL CIRCULATION SEEN IN DATA FROM THE SAN JUAN WSR-88D RADAR. THIS TILTED STRUCTURE IS CONSISTENT WITH SOUTHERLY TO SOUTHWESTERLY VERTICAL SHEAR OF 5 TO 10 KT SHOWN OVER THE CYCLONE BY THE UW-CIMSS AND SHIPS MODEL ANALYSES. IN ADDITION...THE DROPSONDE DATA SHOWED DRY AIR IN THE MID LEVELS OF THE ATMOSPHERE AROUND GABRIELLE. GIVEN THE ENVIRONMENT...THE SOMEWHAT DISORGANIZED STATE OF THE CIRCULATION...POSSIBLE LAND INTERACTION...AND THE PRESENCE OF THE LARGE AREA OF DISTURBED WEATHER NORTHEAST OF GABRIELLE...NOT MUCH STRENGTHENING IS EXPECTED IN THE SHORT TERM. THE NHC FORECAST IS IN GOOD AGREEMENT WITH THE INTENSITY CONSENSUS AIDS FOR THE FIRST 24 HOURS. IT IS ENTIRELY POSSIBLE THAT THE LOW-LEVEL CIRCULATION WILL DISSIPATE IN THE NEXT 24 HOURS. ASSUMING THAT GABRIELLE SURVIVES...ONLY LIMITED INTENSIFICATION IS SHOWN LATER IN THE PERIOD AS THE SHEAR INCREASES DRAMATICALLY AHEAD OF AN UPPER-LEVEL TROUGH. THE NHC FORECAST REMAINS BELOW MUCH OF THE INTENSITY GUIDANCE LATE IN THE PERIOD...WHICH SEEMS TOO HIGH GIVEN THE INCREASE IN SHEAR. THE NHC FORECAST IS ALSO SUPPORTED BY THE GLOBAL MODELS...WHICH DO NOT SHOW MUCH INTENSIFICATION AFTER GABRIELLE MOVES NORTH OF THE CARIBBEAN SEA.

THE LOW-LEVEL CENTER HAS BEEN DIFFICULT TO LOCATE...BUT THE GLOBAL HAWK DROPSONDE DATA SUGGEST IT REMAINS SOUTHWEST OF THE MID-LEVEL CENTER SEEN IN RADAR IMAGERY. GIVEN THE DIFFICULTY LOCATING THE CENTER...THE INITIAL MOTION ESTIMATE IS AN UNCERTAIN 320/7. THE SYNOPTIC-SCALE TRACK REASONING HAS NOT CHANGED...AS GABRIELLE IS EXPECTED TO MOVE GENERALLY NORTHWESTWARD DURING THE NEXT DAY OR SO...FOLLOWED A TURN TOWARD THE NORTH BY 72 HOURS AS THE CYCLONE MOVES AROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE. LATE IN THE PERIOD AN ACCELERATION TOWARD THE NORTHEAST IS EXPECTED AHEAD OF AN UPPER-LEVEL TROUGH OVER THE WESTERN ATLANTIC. THE NHC TRACK FORECAST IS LARGELY AN UPDATE OF THE PREVIOUS ONE THROUGH 72 HOURS AND LIES A LITTLE TO THE RIGHT OF MOST OF THE MODEL GUIDANCE DURING THIS PERIOD. AT DAYS 4 AND 5 THE NHC FORECAST IS A LITTLE TO THE RIGHT OF THE PREVIOUS ONE AND IS CLOSE TO THE GFS ENSEMBLE MEAN AND FSU SUPERENSEMBLE.

REGARDLESS OF THE EXACT TRACK OF THE CENTER OF GABRIELLE DURING THE NEXT 24 HOURS...THE MAIN THREAT WILL BE HEAVY RAINFALL OVER PORTIONS OF PUERTO RICO AND THE DOMINICAN REPUBLIC.


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