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| #38941 (Received by flhurricane at: 4:52 PM 11.Aug.2005) TCDAT4 TROPICAL STORM IRENE DISCUSSION NUMBER 29 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 PM EDT THU AUG 11 2005 THE OVERALL CONVECTIVE PATTERN OF IRENE HAS CHANGED LITTLE TODAY. THE SYSTEM IS ELONGATED WEST-NORTHWEST TO EAST-SOUTHEAST AND THE BEST ESTIMATE OF THE SURFACE CENTER LOCATION...BASED ON CURVED LOW-LEVEL CLOUD LINES AND CONTINUITY...IS NEAR THE WESTERN EDGE OF THE CLOUD SHIELD AND NOT FARTHER EAST INTO THE DEEP CONVECTION. SATELLITE INTENSITY ESTIMATES ARE 35 KT...45 KT...AND 55 KT FROM TAFB...SAB...AND AFWA...RESPECTIVELY. THE INITIAL INTENSITY IS BEING MAINTAINED AT 45 KT FOR THIS ADVISORY...BUT IT COULD BE A LITTLE HIGHER. THE INITIAL MOTION ESTIMATE IS 300/13...BASED MAINLY ON CONTINUITY AND AND PARTLY ON THE MODELS INDICATING A GRADUAL DECREASE IN FORWARD SPEED. THERE HAS NOT BEEN MUCH CHANGE IN THE MODEL FORECAST TRACKS OR THE MODELS' HANDLING OF THE SUBTROPICAL RIDGE TO THE NORTH OF IRENE AND THE ALLEGED WEAKENING OF THE WESTERN PORTION OF THE RIDGE AFTER 72 HOURS. HOWEVER...IT SHOULD BE NOTED THAT NONE OF THE MODELS CREATE A WEAKNESS IN THE RIDGE DUE TO A SHORTWAVE TROUGH MOVING INTO THE MEAN RIDGE POSITION. THE NOGAPS AND UKMET MODELS ERODE THE RIDGE AS A RESULT OF AMPLIFYING IRENE TO A SIZE NEARLY THE SAME AS THAT OF THE SYNOPTIC SCALE HIGH PRESSURE CENTER NEAR BERMUDA...WHICH RESULTS IN IRENE TURNING SHARPLY NORTHWARD ALONG 70W LONGITUDE BY 72 HOURS. THAT SCENARIO SEEMS LESS LIKELY...GIVEN THE CURRENT SIZE AND STRENGTH OF THE BERMUDA RIDGE AS NOTED IN WATER VAPOR IMAGERY AND UPPER-AIR DATA. THE GFS HAS CONTINUED ITS TREND OF DISSIPATING IRENE WITHIN 48 HOURS AND MAINTAINING AN EAST- WEST ORIENTED MID-LEVEL RIDGE FROM BERMUDA WESTWARD TO THE CAROLINAS. THE OFFICIAL FORECAST IS CLOSE TO THE GFDL AND GFDN SOLUTIONS...WHICH MOVE IRENE FARTHER WEST AND CLOSER TO THE OUTER BANKS OF NORTH CAROLINA...AND DECREASE THE FORWARD SPEED TO ONLY 5 KT AFTER 72 HOURS. THE OFFICIAL FORECAST IS NEARLY IDENTICAL TO THE PREVIOUS TRACK AND REMAINS NEAR THE LEFT EDGE OF THE MODEL GUIDANCE ENVELOPE. IT SHOULD BE POINTED OUT THAT WEAK STEERING CURRENTS LESS THAN 10 KT OFTENTIMES RESULTS IN ERRATIC MOTION...SO A POSSIBLE U.S. LANDFALL BY DAY 5 CAN NOT BE RULED OUT AT THIS TIME. AN UPPER-LOW TO THE SOUTH OF IRENE HAS BEEN ADVECTING DRY MID-LEVEL INTO THE SOUTHERN PORTION OF THE CYCLONE TODAY...WHICH HAS CAUSED THE CONVECTION TO ERODE AND WEAKEN AT TIMES. HOWEVER...WATER VAPOR IMAGERY INDICATES THE LOW HAS BEEN WEAKENING AND THAT THE MID- TO UPPER-LEVEL FLOW IS GRADUALLY BECOMING SOUTHEASTERLY. THEREFORE... DEEP CONVECTION HAS A BETTER CHANCE OF DEVELOPING LATER TONIGHT AND IRENE COULD BECOME A HURRICANE BY THIS TIME TOMORROW. LATER IN THE FORECAST PERIOD...DRY MID-LEVEL AIR REMAINS THE PRIMARY INHIBITING FACTOR. THE SHIPS MODEL HAS NOW DECREASED THE PEAK INTENSITY TO LESS THAN 70 KT DUE TO STRONG LOW-LEVEL ANTICYCLONIC FLOW SURROUNDING THE CYCLONE AS IT MOVES INTO A STRONGER RIDGE. THE OFFICIAL INTENSITY FORECAST REMAINS ABOVE THE SHIPS MODEL SINCE THE GFS-BASED SHIPS MODEL DOES NOT HOLD ONTO IRENE AFTER 48 HOURS... WHICH RESULTS IN AN ARTIFICIALLY STRONGER RIDGE IN THE GFS MODEL. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 11/2100Z 25.7N 63.9W 45 KT 12HR VT 12/0600Z 26.7N 65.6W 55 KT 24HR VT 12/1800Z 27.8N 67.6W 60 KT 36HR VT 13/0600Z 28.5N 69.1W 65 KT 48HR VT 13/1800Z 29.1N 70.5W 70 KT 72HR VT 14/1800Z 31.0N 72.5W 75 KT 96HR VT 15/1800Z 33.0N 73.5W 75 KT 120HR VT 16/1800Z 35.0N 74.5W 75 KT |
