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| #452134 (Received by flhurricane at: 11:03 PM 21.Aug.2011) TCDAT4 TROPICAL STORM IRENE DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011 1100 PM AST SUN AUG 21 2011 IRENE HAS CONTINUED TO BECOME BETTER ORGANIZED OVER THE PAST FEW HOURS...AS THE CIRCULATION HAS BECOME BETTER DEFINED AND RADIAL VELOCITIES HAVE INCREASED ON THE SAN JUAN DOPPLER RADAR. THERE HAS ALSO BEEN A MARKED INCREASE IN DEEP CONVECTION NEAR THE CENTER ON SATELLITE IMAGERY. DATA FROM THE AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INDICATE THAT THE CENTRAL PRESSURE HAS FALLEN TO 993 MB. THE INITIAL INTENSITY IS SET AT 60 KT BASED ON A PEAK BELIEVABLE SFMR VALUE OF 61 KT AND AN EXPANDING AREA OF 65 TO 70 KT DOPPLER VELOCITIES AT 5500 TO 6000 FEET ON THE SAN JUAN RADAR. UPPER-LEVEL OUTFLOW IS WELL ESTABLISHED IN ALL QUADRANTS EXCEPT THE SOUTHEAST AND ATMOSPHERIC AND OCEANIC CONDITIONS APPEAR FAVORABLE FOR INTENSIFICATION...SO THE INTENSITY OF IRENE WILL LARGELY BE DETERMINED BY HOW MUCH THE CYCLONE INTERACTS WITH LAND IN THE NEXT 24 TO 36 HOURS. SOME INTENSIFICATION IS STILL EXPECTED IN THE NEXT FEW HOURS...AND IRENE COULD BECOME A HURRICANE BEFORE MOVING OVER PUERTO RICO. THE CYCLONE IS THEN EXPECTED TO MOVE ACROSS THE NORTHERN PORTION OF HISPANIOLA BETWEEN 24 AND 36 HOURS...AND SOME WEAKENING IS INDICATED DURING THAT TIME. HOWEVER...IF IRENE MOVES NORTH OF HISPANIOLA...IT COULD BE STRONGER THAN INDICATED HERE IN 36 TO 48 HOURS...OR WEAKER THAN FORECAST IF IT MOVES OVER MORE OF THE HIGH TERRAIN OF THAT ISLAND. BEYOND 48 HOURS GRADUAL STRENGTHENING IS EXPECTED AS IRENE MOVES OVER WARM WATERS NEAR THE BAHAMAS...BUT THE UNCERTAINTY REMAINS VERY HIGH LATE IN THE PERIOD DUE TO THE POTENTIAL FOR LAND INTERACTION IN THE SHORT TERM. WHILE THE FORWARD MOTION OF THE CENTER APPEARED TO HAVE SLOWED ON RADAR IMAGERY AS THE CYCLONE ORGANIZED DURING THE EVENING HOURS... THE LONG TERM INITIAL MOTION ESTIMATE IS TOWARD 285 DEGREES AT ABOUT 13 KNOTS. THE SUBTROPICAL RIDGE CURRENTLY SITUATED NORTH OF THE CYCLONE WILL BREAK DOWN OVER THE NEXT DAY OR SO AS A LONGWAVE MID-LEVEL TROUGH AMPLIFIES OVER THE WESTERN ATLANTIC. THE TROUGH BEGINS TO LIFT OUT AFTER ABOUT 36 HOURS OR SO...BUT LEAVES A PRONOUNCED WEAKNESS IN THE RIDGE OVER THE SOUTHWESTERN ATLANTIC AND THE BAHAMAS. MOST OF THE GUIDANCE...WITH THE EXCEPTION OF THE GFDL AND UKMET...ARE IN RELATIVELY GOOD AGREEMENT ON TURNING IRENE NORTHWESTWARD INTO THE BREAK IN THE RIDGE IN 2 OR 3 DAYS. HOWEVER...SIGNIFICANT SPREAD REMAINS IN THE SHARPNESS OF THIS TURN IN THE MODEL SUITE. FOR THIS CYCLE THE CONSENSUS TRACK MODELS HAVE SHIFTED BACK A LITTLE TO THE LEFT...WITH THE GFS...ECMWF...AND HWRF MODELS ON THE EAST SIDE OF THE GUIDANCE ENVELOPE AND THE AFOREMENTIONED UKMET AND GFDL FARTHER WEST. THE OFFICIAL FORECAST HAS BEEN NUDGED TO THE RIGHT...TOWARD BUT STILL TO THE LEFT OF THE MODEL CONSENSUS...AND LIES CLOSE TO THE FIM MODEL PROVIDED BY THE HURRICANE FORECAST IMPROVEMENT PROJECT. IT IS IMPORTANT NOT TO FOCUS ON THE EXACT FORECAST TRACK... ESPECIALLY AT DAYS 4 TO 5...SINCE THE MOST RECENT 5-YEAR AVERAGE ERRORS AT THOSE FORECAST TIMES ARE 200 AND 250 MILES... RESPECTIVELY. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 17.9N 65.5W 60 KT 70 MPH 12H 22/1200Z 18.6N 67.7W 60 KT 70 MPH 24H 23/0000Z 19.4N 70.1W 60 KT 70 MPH...INLAND 36H 23/1200Z 20.1N 72.1W 50 KT 60 MPH...OVER WATER 48H 24/0000Z 20.9N 73.9W 55 KT 65 MPH 72H 25/0000Z 23.5N 77.0W 65 KT 75 MPH 96H 26/0000Z 26.5N 79.5W 80 KT 90 MPH 120H 27/0000Z 30.0N 81.5W 75 KT 85 MPH...INLAND $$ FORECASTER BRENNAN |
