000 WTPZ45 KNHC 140251 TCDEP5 TROPICAL STORM ISELLE DISCUSSION NUMBER 3 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP102008 800 PM PDT WED AUG 13 2008 AN ASCAT PASS AT 1630Z THIS AFTERNOON PLACED THE CENTER OF ISELLE ON THE NORTHEASTERN EDGE OF A SMALL BLOB OF CONVECTION...FARTHER NORTH THAN PREVIOUSLY ESTIMATED...AND LACKING ANY EVIDENCE TO THE CONTRARY I'M ASSUMING THAT RELATIVE LOCATION STILL HOLDS. THIS REQUIRES A NORTHWARD ADJUSTMENT OF THE PREVIOUS TRACK WITH AN INITIAL MOTION ESTIMATE OF 295/10. ISELLE IS CURRENTLY LOCATED TO THE SOUTH OF MID- TO UPPER-LEVEL HIGH PRESSURE OVER NORTH-CENTRAL MEXICO...BUT THE TRACK FORECAST IS UNUSUALLY DIFFICULT...AS ISELLE IS IN A HIGH-SHEAR ENVIRONMENT AND THE GUIDANCE IS HIGHLY DIVERGENT. MOST OF THE GUIDANCE CALLS FOR A CONTINUED TRACK TO THE WEST-NORTHWEST OR NORTHWEST FOR AT LEAST THE NEXT COUPLE OF DAYS AS ISELLE APPROACHES THE WESTERN PERIPHERY OF THE AFOREMENTIONED HIGH PRESSURE. THE GFDL...HOWEVER...CALLS FOR ISELLE TO SUCCUMB TO THE SHEAR WITHIN A DAY OR SO...TAKING ITS MID-LEVEL VORTICITY WESTWARD AND ITS LOW-LEVEL REMNANTS NORTHWARD TOWARD THE BAJA PENINSULA. THE LATTER TRACK SCENARIO IS BEING DISCOUNTED FOR NOW...NOT SO MUCH BECAUSE IT IS UNLIKELY...BUT BECAUSE IF SUCH A TRACK DOES MATERIALIZE THERE SHOULD BE VERY LITTLE OF ISELLE LEFT. IF ISELLE IS STILL A TROPICAL CYCLONE IN 48-72 HOURS...IT WILL HAVE HAD TO FOLLOW A DEEPER LAYER FLOW AND THAT IS THE ASSUMPTION REFLECTED IN THE TRACK GIVEN BELOW. THE OFFICIAL FORECAST HAS BEEN SHIFTED NORTHWARD OF THE PREVIOUS ADVISORY TRACK AND IS IN BEST AGREEMENT WITH THE GFS AND MEDIUM BAM THROUGH 48 HOURS...AND SHOWS RELATIVELY LITTLE MOTION AFTER THAT IN DEFERENCE TO THE MODEL SPREAD. IT SHOULD BE NOTED THAT THE MODEL CONSENSUS LIES EVEN FARTHER TO THE NORTH AND EAST OF THE OFFICIAL FORECAST. THE INITIAL INTENSITY REMAINS 35 KT...WITH DVORAK INTENSITY ESTIMATES OF 35 AND 45 KT FROM SAB AND TAFB...RESPECTIVELY. NORTHEASTERLY OR EASTERLY SHEAR IS FORECAST BY THE GLOBAL MODELS TO CONTINUE FOR MOST OF THE FIVE-DAY FORECAST PERIOD...AND NONE OF THE AVAILABLE INTENSITY GUIDANCE SUGGESTS MUCH STRENGTHENING. THE OFFICIAL FORECAST CALLS FOR VERY LITTLE CHANGE IN STRENGTH UNTIL AFTER 72 HOURS WHEN ISELLE IS EXPECTED TO CROSS THE 26C SST ISOTHERM. AS NOTED ABOVE...IT IS QUITE POSSIBLE THAT ISELLE WILL NOT LAST THAT LONG. FORECAST POSITIONS AND MAX WINDS INITIAL 14/0300Z 17.4N 107.7W 35 KT 12HR VT 14/1200Z 17.9N 108.9W 40 KT 24HR VT 15/0000Z 18.4N 110.3W 40 KT 36HR VT 15/1200Z 19.0N 111.3W 40 KT 48HR VT 16/0000Z 19.8N 112.5W 40 KT 72HR VT 17/0000Z 20.5N 114.0W 40 KT 96HR VT 18/0000Z 21.0N 115.5W 35 KT 120HR VT 19/0000Z 21.0N 117.0W 30 KT $$ FORECASTER FRANKLIN