000 WTPZ45 KNHC 141433 TCDEP5 TROPICAL STORM ISELLE DISCUSSION NUMBER 5 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP102008 800 AM PDT THU AUG 14 2008 ANOTHER IMPRESSIVE NOCTURNAL BURST OF CONVECTION DEVELOPED WELL-DEFINED MID- AND LOW-LEVEL CIRCULATION CENTERS AS NOTED IN A 14/0921Z TRMM OVERPASS. HOWEVER...SINCE THAT TIME...THE OVERALL CONVECTIVE PATTERN HAS DETERIORATED SOMEWHAT DUE TO INCREASING NORTHEASTERLY SHEAR. THE INITIAL INTENSITY WAS INCREASED TO 45 KT AT 12Z AND FOR THE ADVISORY TIME BASED ON CONSENSUS T3.0 SATELLITE ESTIMATES FROM TAFB AND SAB. THIS MAY BE A LITTLE GENEROUS AT THIS TIME...BUT ISELLE HAS HAD A HISTORY OF REDEVELOPING DEEP CONVECTION IN THE FACE OF OTHERWISE HOSTILE CONDITIONS. THE CENTER LOCATION IS BASED ON EXTRAPOLATING THE VORTEX TILT TO THE NORTHEAST NOTED IN TRMM IMAGERY AND USING THE STORM MOTION FROM THE PREVIOUS ADVISORY. TROPICAL STORM ISELLE IS EXPECTED TO CONTINUE IN A GENERAL WEST-NORTHWESTWARD MOTION THROUGH 72 HOURS...AND THEN TURN WESTWARD AFTER THAT AS A WEAKENING TROPICAL CYCLONE OR REMNANT LOW THAT IS STEERED BY THE LOW-LEVEL EASTERLIES NORTH OF THE SYSTEM. THE MAJORITY OF THE NHC MODEL GUIDANCE SUPPORTS THIS SCENARIO...EXCEPT FOR THE GFDL AND A UKMET MODELS WHICH TAKE THE CYCLONE SLOWLY NORTH-NORTHWESTWARD. THE OFFICIAL FORECAST TRACK LEANS MORE TOWARD A WESTWARD MOTION LATE IN THE PERIOD AS ISELLE IS EXPECTED TO COME UNDER THE INFLUENCE OF A LARGER CYCLONIC GYRE CURRENTLY CENTERED ABOUT 650 NMI TO ITS WEST-SOUTHWEST. THE OFFICIAL FORECAST TRACK IS A LITTLE SOUTH OF THE PREVIOUS TRACK AND SIMILAR TO THE NHC MODEL CONSENSUS. VERTICAL SHEAR IS EXPECTED REMAIN BETWEEN 25 TO 30 KT THROUGH 96 HOURS. THESE UNFAVORABLE UPPER-LEVEL CONDITIONS...COUPLED WITH DECREASING SEA-SURFACE TEMPERATURES...ARE EXPECTED TO INDUCE GRADUAL WEAKENING BY 24-36 HOURS...OF NOT SOONER. THE OFFICIAL INTENSITY FORECAST IS SIMILAR TO BUT SLIGHTLY LESS THAN THE SHIPS INTENSITY MODEL. FORECAST POSITIONS AND MAX WINDS INITIAL 14/1500Z 17.8N 109.4W 45 KT 12HR VT 15/0000Z 18.4N 110.5W 45 KT 24HR VT 15/1200Z 19.1N 111.8W 45 KT 36HR VT 16/0000Z 19.7N 112.8W 40 KT 48HR VT 16/1200Z 20.2N 113.9W 40 KT 72HR VT 17/1200Z 20.8N 115.4W 35 KT 96HR VT 18/1200Z 21.0N 116.5W 30 KT 120HR VT 19/1200Z 21.0N 118.0W 25 KT $$ FORECASTER STEWART/BEVEN