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| #536855 (Received by flhurricane at: 11:05 AM 25.Aug.2012) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012 1100 AM EDT SAT AUG 25 2012 ISAAC HAS BEEN SOMEWHAT DISRUPTED BY ITS PASSAGE ACROSS HAITI. THE LAST REPORT FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT SHOWED THAT THE CENTER PRESSURE HAD RISEN TO 998 MB...AND THAT THE STRONGEST FLIGHT-LEVEL WINDS WERE 60-70 N MI FROM THE CENTER. THESE WINDS SUPPORT AN INITIAL INTENSITY OF 50 KT. THE CONVECTIVE PATTERN IS DISORGANIZED...WITH LARGE CONVECTIVE BANDS SOUTH OF HISPANIOLA AND LITTLE CONVECTION CURRENTLY NEAR THE CENTER. IT SHOULD BE NOTED THAT ISAAC IS AT THE EASTERN END OF A LARGE AND ELONGATED LOW PRESSURE SYSTEM OVER THE NORTHWESTERN CARIBBEAN SEA...WHICH IS CREATING A LARGE AREA OF STRONG EASTERLY LOW-LEVEL WINDS WELL TO THE NORTHWEST OF THE CENTER. THE LOW-LEVEL CENTER HAS JOGGED WESTWARD DURING THE PAST FEW HOURS...AND THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 315/15. IN THE SHORT-TERM...A MID/UPPER-LEVEL TROUGH OVER THE EASTERN UNITED STATES IS LIFTING OUT TO THE NORTHEAST AS A UPPER-LEVEL LOW MOVES SOUTHWARD ACROSS THE CENTRAL GULF OF MEXICO. THIS EVOLUTION... COMBINED WITH THE LOW-LEVEL RIDGE NORTH OF ISAAC...SHOULD STEER THE CYCLONE GENERALLY NORTHWESTWARD NEAR THE NORTH COAST OF CUBA AND THE FLORIDA KEYS DURING THE NEXT 48 HR AS INDICATED BY THE TIGHTLY- CLUSTERED MODEL GUIDANCE. AFTER THAT...THE CYCLONE IS EXPECTED TO MOVE AROUND THE WESTERN END OF THE SUBTROPICAL RIDGE AND TURN MORE NORTHWARD OVER THE EASTERN GULF OF MEXICO. WHILE THE GUIDANCE GENERALLY AGREES WITH THIS SCENARIO...THERE IS A SPREAD IN POTENTIAL LANDFALL LOCATIONS BETWEEN THE EASTERN FLORIDA PANHANDLE AND SOUTHEASTERN LOUISIANA...WHICH LOWERS CONFIDENCE IN THE FINAL LANDFALL POINT. THE NEW FORECAST TRACK FOLLOWS THE CENTER OF THE GUIDANCE ENVELOPE AND THE CONSENSUS MODELS IN CALLING FOR LANDFALL IN THE WESTERN FLORIDA PANHANDLE BETWEEN 72-96 HR. LITTLE CHANGE IN STRENGTH IS EXPECTED TODAY DUE TO A COMBINATION OF LAND INTERACTION...SOME WESTERLY SHEAR...AND THE CURRENT LACK OF CONVECTION. THE SHEAR IS FORECAST TO DECREASE AS THE CENTER MOVES AWAY FROM LAND...WOULD SHOULD ALLOW FOR STRENGTHENING AFTER 12 HR. THE NEW INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS FORECAST THROUGH 48 HR...THEN CALLS FOR A LITTLE MORE STRENGTHENING THAN THE PREVIOUS FORECAST OVER THE NORTHERN GULF. THE CYCLONE SHOULD WEAKEN TO A DEPRESSION AFTER LANDFALL ON THE NORTHERN GULF COAST. THE NEW NHC FORECAST NECESSITATES THE ISSUANCE OF ADDITIONAL TROPICAL STORM WARNINGS AND WATCHES FOR PORTIONS OF THE FLORIDA PENINSULA. IT IS IMPORTANT NOT TO FOCUS ON THE EXACT TRACK OF ISAAC DUE TO THE UNCERTAINTIES IN THE FORECAST AND THE FACT THAT ISAAC HAS A LARGE AREA OF TROPICAL STORM FORCE WINDS ASSOCIATED WITH IT. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 20.1N 74.6W 50 KT 60 MPH 12H 26/0000Z 21.7N 76.4W 50 KT 60 MPH 24H 26/1200Z 23.3N 79.0W 55 KT 65 MPH 36H 27/0000Z 24.7N 81.3W 65 KT 75 MPH 48H 27/1200Z 25.9N 83.1W 75 KT 85 MPH 72H 28/1200Z 28.5N 85.5W 85 KT 100 MPH 96H 29/1200Z 31.5N 86.5W 65 KT 75 MPH...INLAND 120H 30/1200Z 33.0N 86.0W 30 KT 35 MPH...INLAND $$ FORECASTER BEVEN |
