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| #535986 (Received by flhurricane at: 10:50 PM 21.Aug.2012) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012 1100 PM AST TUE AUG 21 2012 THERE HAS BEEN NO AERIAL RECONNAISSANCE INTO ISAAC RECENTLY...BUT THE CLOUD PATTERN OF THE STORM HAS NOT BECOME SIGNIFICANTLY BETTER ORGANIZED OVER THE PAST FEW HOURS. THE INTENSITY IS HELD AT 35 KT WHICH IS IN AGREEMENT WITH A DVORAK INTENSITY ESTIMATE FROM TAFB. ANOTHER HURRICANE HUNTER AIRCRAFT IS SCHEDULED TO INVESTIGATE ISAAC AT AROUND 0600 UTC. THERE IS SOME SPREAD IN CENTER FIXES...BUT BASED ON GEOSTATIONARY AND MICROWAVE IMAGERY THE INITIAL MOTION REMAINS ABOUT THE SAME...275/16. A WELL-DEFINED MID-TROPOSPHERIC RIDGE TO THE NORTH OF THE TROPICAL CYCLONE SHOULD MAINTAIN THE WESTWARD TRACK FOR THE NEXT COUPLE OF DAYS. LATER IN THE FORECAST PERIOD...GLOBAL MODELS DEPICT ISAAC NEARING A BREAK IN THE RIDGE NEAR FLORIDA. CONSEQUENTLY THE FORWARD SPEED IS LIKELY TO SLOW...AND THE HEADING TO GRADUALLY TURN TOWARD THE RIGHT...IN THE LATTER HALF OF THE FORECAST PERIOD. ALTHOUGH THE GFS HAS SHIFTED JUST A BIT TO THE NORTH OF ITS PREVIOUS FORECAST...OVERALL THE TRACK GUIDANCE HAS NOT CHANGED MUCH SINCE THE PREVIOUS CYCLE. THE OFFICIAL FORECAST IS VERY CLOSE TO THE LATEST HWRF TRACK AND NOT FAR FROM THE DYNAMICAL CONSENSUS. WATER VAPOR IMAGERY SUGGESTS THAT EAST-NORTHEASTERLY SHEAR IS CURRENTLY AFFECTING ISAAC...BUT THE DYNAMICAL GUIDANCE PREDICTS A DECREASE IN SHEAR WITH UPPER-LEVEL ANTICYCLONIC FLOW OVER THE TROPICAL CYCLONE DURING THE NEXT SEVERAL DAYS. THEREFORE STRENGTHENING IS ANTICIPATED...AND ISAAC COULD BECOME A HURRICANE WITHIN THE NEXT COUPLE OF DAYS. LATER ON...THE INTENSITY FORECAST IS COMPLICATED BY HOW MUCH THE CORE OF THE SYSTEM INTERACTS WITH THE MOUNTAINOUS LAND MASSES OF HISPANIOLA AND EASTERN CUBA. THE OFFICIAL WIND SPEED FORECAST IS FAIRLY CLOSE TO THE STATISTICAL- DYNAMICAL GUIDANCE FOR THE FIRST 48 HOURS AND THEN TRIES TO TAKE INTO ACCOUNT THE LAND INTERACTION. NEEDLESS TO SAY...THERE IS CONSIDERABLE UNCERTAINTY IN THE INTENSITY OF ISAAC IN THE 3 TO 5 DAY TIME FRAME. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 15.6N 55.6W 35 KT 40 MPH 12H 22/1200Z 15.8N 58.1W 40 KT 45 MPH 24H 23/0000Z 16.2N 61.2W 45 KT 50 MPH 36H 23/1200Z 16.7N 64.3W 55 KT 65 MPH 48H 24/0000Z 17.1N 67.1W 65 KT 75 MPH 72H 25/0000Z 18.3N 72.0W 65 KT 75 MPH...INLAND 96H 26/0000Z 20.5N 76.0W 70 KT 80 MPH...INLAND 120H 27/0000Z 23.0N 79.0W 70 KT 80 MPH...OVER WATER $$ FORECASTER PASCH |
