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| #537078 (Received by flhurricane at: 5:02 AM 26.Aug.2012) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 21 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012 500 AM EDT SUN AUG 26 2012 WHILE THE SATELLITE PRESENTATION OF ISAAC REMAINS SOMEWHAT UNIMPRESSIVE...WITH DEEP CONVECTION LIMITED TO A RATHER SMALL AREA NORTH AND EAST OF THE CENTER...AIRCRAFT DATA SUGGEST THAT THE CYCLONE HAS STRENGTHENED A LITTLE OVERNIGHT. PEAK 850-MB FLIGHT-LEVEL WINDS OF 69 KT IN THE NORTHEAST QUADRANT ADJUST TO AN INTENSITY OF 55 KT...WHICH IS THE VALUE FOR THIS ADVISORY. AIRCRAFT DATA INDICATE THAT THE MINIMUM PRESSURE HAS ALSO DROPPED A LITTLE... TO 995 MB. ISAAC STILL APPEARS TO BE BATTLING THE EFFECTS OF SOME LOW- TO MID-LEVEL DRY AIR IN THE WESTERN SIDE OF THE CIRCULATION... AS SEEN IN UPPER-AIR DATA FROM MIAMI AND KEY WEST. ASIDE FROM THIS...HOWEVER...CONDITIONS APPEAR FAVORABLE FOR STEADY INTENSIFICATION AS THE CYCLONE MOVE ACROSS THE FLORIDA STRAITS AND INTO THE EASTERN GULF OF MEXICO. WATERS ALONG THE FORECAST TRACK ARE VERY WARM...AND UPPER-LEVEL WINDS ARE FORECAST BY THE GLOBAL MODELS TO BECOME CONDUCIVE FOR STRENGTHENING. THE NEW NHC FORECAST IS SIMILAR TO THE PREVIOUS ONE...BUT SHOWS A LITTLE SLOWER RATE OF STRENGTHENING FOR THE FIRST COUPLE OF DAYS...TRENDING TOWARD THE LATEST CYCLE OF GUIDANCE. THE INITIAL MOTION ESTIMATE IS 305/16...A LITTLE FASTER THAN BEFORE. AS THE UPPER-LOW TO THE WEST OF ISAAC RETREATS WESTWARD AND THE CLOSED LOW OVER THE MID-ATLANTIC STATES LIFTS NORTHWARD...ISAAC WILL MOVE GENERALLY WEST-NORTHWESTWARD OR NORTHWESTWARD AROUND THE WESTERN PERIPHERY OF THE ATLANTIC SUBTROPICAL RIDGE FOR THE NEXT 48 HOURS. BEYOND THAT TIME...THERE IS CONSIDERABLE SPREAD IN THE MODEL GUIDANCE. MOST OF THE MODELS...WITH THE EXCEPTION OF THE UKMET...NO LONGER SHOW ISAAC RECURVING AHEAD OF A SHORTWAVE TROUGH MOVING EASTWARD ACROSS THE EASTERN UNITED STATES. ONE CAMP OF GUIDANCE... INCLUDING THE GFS...HWRF...AND GFS ENSEMBLE MEAN SHOW A NORTHWESTWARD TRACK TOWARD THE CENTRAL GULF COAST AS THE SHORTWAVE BYPASSES ISAAC COMPLETELY. THE ECMWF AND GFDL SHOW HAVE MORE OF A WEAKNESS IN THE RIDGE AND SHOW ISAAC TURNING NORTHWARD TOWARD THE EASTERN GULF COAST. THE NEW NHC FORECAST IS FASTER THAN THE PREVIOUS ONE THROUGH MUCH OF THE PERIOD...BUT IS ALONG THE PREVIOUS TRACK THROUGH 36 HOURS. AFTER THAT TIME...A SIGNIFICANT WESTWARD ADJUSTMENT WAS MADE...AND THE NEW NHC TRACK IS CLOSE TO THE TVCA MULTI-MODEL CONSENSUS AND THE ECMWF...SHOWING A NORTH-NORTHWESTWARD AND NORTHWARD TURN AT DAYS 3 THROUGH 5. GIVEN THE LARGE SPREAD IN THE GUIDANCE...CONFIDENCE IN THE LONG-RANGE TRACK FORECAST IS QUITE LOW...AND IT IS TOO EARLY TO DETERMINE EXACTLY WHERE AND WHEN ISAAC WILL MAKE LANDFALL ALONG THE GULF COAST. FURTHER ADJUSTMENTS TO THE TRACK MAY BE NECESSARY LATER TODAY. THROUGHOUT THE PERIOD...IT IS IMPORTANT NOT TO FOCUS ON THE EXACT FORECAST TRACK SINCE SIGNIFICANT HAZARDS EXTEND WELL AWAY FROM THE CENTER. GIVEN THE NEW NHC FORECAST...A HURRICANE WATCH HAS BEEN ISSUED FOR A PORTION OF THE NORTHERN GULF COAST. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 23.1N 79.0W 55 KT 65 MPH 12H 26/1800Z 24.2N 80.8W 60 KT 70 MPH 24H 27/0600Z 25.5N 83.1W 65 KT 75 MPH 36H 27/1800Z 26.9N 84.9W 70 KT 80 MPH 48H 28/0600Z 28.1N 86.5W 80 KT 90 MPH 72H 29/0600Z 30.2N 88.0W 90 KT 105 MPH 96H 30/0600Z 32.0N 88.5W 40 KT 45 MPH...INLAND 120H 31/0600Z 34.0N 88.5W 25 KT 30 MPH...INLAND $$ FORECASTER BRENNAN |
