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| #113514 (Received by flhurricane at: 11:11 PM 28.Sep.2006) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 6 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL092006 1100 PM EDT THU SEP 28 2006 THE OVERALL CLOUD PATTERN OF ISAAC HAS CHANGED VERY LITTLE THIS EVENING. A CONVECTIVE BAND HAS WRAPPED AROUND THE WEST AND SOUTHWEST SIDE OF THE CIRCULATION...WHICH HAS MADE IT A LITTLE MORE DIFFICULT TO DETERMINE THE CENTER LOCATION WITH INFRARED IMAGERY. SINCE THE CLOUD TOPS ARE NOT VERY COLD AND THE CONVECTIVE BAND IS RATHER SMALL...DVORAK DATA T-NUMBERS HAVE DECREASED DURING THE PAST 6-12 HOURS. THIS IS A CASE IN WHICH THE DVORAK TECHNIQUE MAY BE UNDERESTIMATING THE INTENSITY...SINCE THE CONVECTION IS NOT VERY DEEP AND THE CYCLONE HAS SOME SUBTROPICAL CHARACTERISTICS. A SHIP WELL SOUTHEAST OF THE CENTER AT 0000 UTC REPORTED 33 KT WINDS...AND STRONGER WINDS COULD BE OCCURRING CLOSER TO THE CENTER. MAKING THAT ASSUMPTION...THE ADVISORY INTENSITY REMAINS 40 KT. THE WIND RADII WERE ALSO EXPANDED SLIGHTLY TO ACCOUNT FOR THE SHIP REPORT. ISAAC HAS BEEN MOVING ALONG THE EASTERN SIDE OF THE MODEL GUIDANCE DURING THE PAST DAY OR SO. MOST OF THE DYNAMICAL MODELS TURN THE CYCLONE WEST-NORTHWESTWARD DURING THE NEXT 12-24 HOURS... IN BETWEEN AN UPPER-LEVEL LOW JUST SOUTH OF THE TROPICAL CYCLONE AND A BUILDING MID-TROPOSPHERIC RIDGE TO THE NORTH. IN ABOUT 2 DAYS...A LARGE DEEP-LAYERED TROUGH MOVING INTO THE WESTERN ATLANTIC SHOULD TURN ISAAC NORTHWARD ALLOWING THE CYCLONE TO RECURVE INTO THE WESTERLIES AHEAD OF THE TROUGH. THERE REMAINS A SIGNIFICANT SPREAD IN THE GUIDANCE DURING RECURVATURE. THE GFS AND NOGAPS REMAIN ON THE WESTERN SIDE OF THE GUIDANCE WHILE THE UKMET IS TO THE EAST. GIVEN THIS LARGE SPREAD...THE OFFICIAL FORECAST TRACK WILL REMAIN CLOSE TO THE MODEL CONSENSUS...WHICH HAPPENS TO BE VERY NEAR THE PREVIOUS NHC FORECAST. ISAAC CONTINUES TO BE AFFECTED BY MODERATE SOUTHERLY SHEAR. HOWEVER...THIS SHEAR IS FORECAST TO DECREASE OVER THE NEXT COUPLE OF DAYS. THE INTENSITY FORECAST IS MORE PROBLEMATIC SINCE ISAAC IS PREDICTED TO MOVE OVER WATERS THAT HAVE BEEN COOLED BY HURRICANES GORDON AND HELENE...WHICH MAY HINDER STRENGTHENING. THEREAFTER...INCREASING SOUTHWESTERLY SHEAR WILL LIKELY BE AN ADDITIONAL INHIBITING FACTOR. THE OFFICIAL FORECAST CALLS FOR ONLY MODEST STRENGTHENING AND IS IN BETWEEN THE SHIPS AND FSU SUPERENSEMBLE. IT SHOULD BE NOTED THAT THE GFDL MAKES ISAAC A HURRICANE IN ABOUT 48 HOURS. HOWEVER...THIS MODEL SOMETIMES OVERESTIMATES THE INTENSIFICATION OF TROPICAL CYCLONES IN A SHEARED ENVIRONMENT. FORECAST POSITIONS AND MAX WINDS INITIAL 29/0300Z 29.2N 55.2W 40 KT 12HR VT 29/1200Z 30.0N 56.0W 40 KT 24HR VT 30/0000Z 30.9N 57.3W 45 KT 36HR VT 30/1200Z 31.9N 58.5W 50 KT 48HR VT 01/0000Z 33.5N 59.6W 50 KT 72HR VT 02/0000Z 38.0N 59.5W 50 KT...BECOMING EXTRATROPICAL 96HR VT 03/0000Z 44.0N 53.5W 50 KT...EXTRATROPICAL 120HR VT 04/0000Z 49.0N 46.0W 50 KT...EXTRATROPICAL $$ FORECASTER BROWN/KNABB |
