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| #536155 (Received by flhurricane at: 4:41 PM 22.Aug.2012) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012 500 PM AST WED AUG 22 2012 SATELLITE...AIRCRAFT...RADAR...AND SURFACE OBSERVATIONS INDICATE THAT ISAAC HAS A BROAD CENTRAL AREA OF LIGHT WINDS WITH MULTIPLE VORTICITY MAXIMA EMBEDDED. THE LOWEST PRESSURES ARE IN AN AREA NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED. HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES. THE INITIAL INTENSITY IS HELD AT 40 KT...ALTHOUGH THE HURRICANE HUNTER AIRCRAFT DATA SUGGESTS THIS COULD BE GENEROUS. THE INITIAL MOTION IS NOW A SOMEWHAT UNCERTAIN 280/19. OTHER THAN THE CENTER POSITION ISSUES...THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST REASONING DURING THE FIRST 48 HR OF THE TRACK FORECAST. THE EAST-WEST ORIENTED SUBTROPICAL RIDGE TO THE NORTH OF ISAAC ALONG 30N LATITUDE IS FORECAST BY ALL OF THE GLOBAL MODELS TO REMAIN INTACT ACROSS FLORIDA AND THE SOUTHEASTERN U.S. DURING THAT TIME. THIS EARLY PART OF THE FORECAST TRACK IS SHIFTED A LITTLE TO THE SOUTH OF THE PREVIOUS FORECAST BASED ON THE OBSERVED POSITION AND MOTION. AFTER 48 HR...THE RIDGE IS EXPECTED TO WEAKEN AS A MID/UPPER-LEVEL TROUGH MOVES ONTO THE EASTERN UNITED STATES. THE GLOBAL MODELS HAVE COME INTO BETTER AGREEMENT SINCE THE LAST ADVISORY...WITH THE NOGAPS AND CANADIAN MODELS SHIFTING TO THE WEST AND THE ECMWF TO THE NORTH. HOWEVER...THE ECMWF CONTINUES TO SHOW A STRONGER RIDGE...AND THUS SHOWS A MORE WESTERLY MOTION THAN THE OTHER MODELS. THE 72-120 HR PORTION OF THE FORECAST TRACK IS SIMILAR TO THE PREVIOUS FORECAST AND LIES NEAR THE CONSENSUS MODELS. THERE REMAINS CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC. WATER VAPOR IMAGERY AND MICROWAVE TOTAL PRECIPITABLE WATER DATA CONTINUE TO SHOW DRY AIR TO THE NORTHEAST OF ISAAC...AND A LACK OF CONVECTION IN THIS AREA SUGGESTS SOME OF THIS AIR IS ENTRAINING INTO THE CYCLONE AND DISRUPTING THE CONVECTIVE STRUCTURE. GIVEN THE CURRENT STRUCTURE...THE NEW INTENSITY FORECAST CALLS FOR LITTLE CHANGE IN STRENGTH FOR THE NEXT 12 HR...FOLLOWED BY INTENSIFICATION INTO A HURRICANE BY 48 HR SIMILAR TO THE PREVIOUS ADVISORY. AFTER 48 HR...THE MAIN FACTOR CONTROLLING THE INTENSITY IS THE AMOUNT OF LAND INTERACTION. THIS LOW-CONFIDENCE PART OF THE INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY BUT ABOVE THE INTENSITY CONSENSUS. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 16.0N 61.2W 40 KT 45 MPH 12H 23/0600Z 16.3N 63.6W 40 KT 45 MPH 24H 23/1800Z 16.7N 66.7W 50 KT 60 MPH 36H 24/0600Z 17.2N 69.4W 60 KT 70 MPH 48H 24/1800Z 17.9N 71.6W 70 KT 80 MPH...INLAND 72H 25/1800Z 20.5N 75.5W 60 KT 70 MPH...INLAND 96H 26/1800Z 23.5N 79.5W 60 KT 70 MPH...OVER WATER 120H 27/1800Z 26.5N 82.5W 70 KT 80 MPH $$ FORECASTER BEVEN |
