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| #453109 (Received by flhurricane at: 5:00 AM 25.Aug.2011) TCDAT4 HURRICANE IRENE DISCUSSION NUMBER 20 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011 500 AM EDT THU AUG 25 2011 A HURRICANE HUNTER AIRCRAFT INVESTIGATING IRENE A FEW HOURS AGO FOUND THAT THE CENTRAL PRESSURE HAD DROPPED TO 950 MB. HOWEVER...FLIGHT-LEVEL AND SFMR-OBSERVED SURFACE WIND DATA SUPPORTED A CURRENT INTENSITY OF NO MORE THAN 100 KT...AND THIS IS PROBABLY GENEROUS. IT IS PRESUMED THAT THE INTENSIFICATION PROCESS WAS HALTED BY AN EYEWALL REPLACEMENT EVENT AS SUGGESTED BY MICROWAVE IMAGERY AND AIRCRAFT OBSERVATIONS. SINCE THE ENVIRONMENT APPEARS TO BE CONDUCIVE WITH WEAK SHEAR...WARM WATERS...AND AN UPPER-LEVEL OUTFLOW ANTICYCLONE OVER THE HURRICANE...THE OFFICIAL FORECAST SHOWS RE-STRENGTHENING WITHIN A DAY OR SO. AFTER A COUPLE OF DAYS...THE SHIPS GUIDANCE SHOWS A SIGNIFICANT INCREASE IN VERTICAL SHEAR...SO A STEADY DECREASE IN INTENSITY IS LIKELY. HOWEVER...SINCE IRENE HAS SUCH A LARGE AND INTENSE CIRCULATION...IT WILL PROBABLY BE RATHER SLOW TO WEAKEN. GIVEN THE LIMITATIONS IN OUR ABILITY TO PREDICT INTENSITY CHANGE...THERE IS SIGNIFICANT UNCERTAINTY AS TO JUST HOW STRONG IRENE WILL BE WHEN IT NEARS THE EASTERN AND NORTHEASTERN U.S. COAST. THE INITIAL MOTION CONTINUES TO BE ABOUT 315/10. THERE HAS BEEN NO CHANGE TO THE TRACK FORECAST REASONING. IRENE SHOULD TURN NORTHWARD AS IT MOVES THROUGH A BREAK IN THE SUBTROPICAL RIDGE DURING THE NEXT 48 HOURS. THEREAFTER...A SHORTWAVE TROUGH MOVING THROUGH THE GREAT LAKES IS LIKELY TO TURN THE HURRICANE TOWARD THE NORTH-NORTHEAST. IN THE LATTER PART OF THE FORECAST PERIOD...GLOBAL MODELS SHOW THE SHORTWAVE BYPASSING IRENE AND THE HURRICANE BECOMING EMBEDDED WITHIN A BROAD AND SLOW-MOVING MID-TROPOSPHERIC TROUGH OVER THE EASTERN UNITED STATES. THIS STEERING PATTERN SHOULD PREVENT IRENE FROM MOVING WELL OFFSHORE OF THE NORTHEASTERN STATES. THE OFFICIAL TRACK FORECAST HAS BEEN SHIFTED VERY SLIGHTLY TO THE WEST OF THE PREVIOUS ONE AND IS ROUGHLY IN THE MIDDLE OF THE GUIDANCE ENVELOPE...WHICH IS BRACKETED BY THE U.K. MET. OFFICE MODEL ON THE EAST AND THE GFDL MODEL ON THE WEST. HURRICANE AND TROPICAL STORM WATCHES ARE NOW NECESSARY FOR A PORTION OF THE SOUTHEAST AND CENTRAL U.S. EAST COAST. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 24.6N 76.2W 100 KT 115 MPH 12H 25/1800Z 26.1N 77.1W 110 KT 125 MPH 24H 26/0600Z 28.2N 77.7W 115 KT 135 MPH 36H 26/1800Z 30.4N 77.5W 110 KT 125 MPH 48H 27/0600Z 32.3N 77.1W 100 KT 115 MPH 72H 28/0600Z 36.3N 75.4W 90 KT 105 MPH 96H 29/0600Z 42.5N 72.5W 60 KT 70 MPH...INLAND 120H 30/0600Z 51.0N 67.0W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ FORECASTER PASCH |
