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| #113557 (Received by flhurricane at: 4:58 AM 29.Sep.2006) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 7 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL092006 500 AM AST FRI SEP 29 2006 ISAAC IS SENDING MIXED STRUCTURAL SIGNALS THIS MORNING. THE CONVECTION HAS BECOME MORE CONSOLIDATED AROUND THE CENTER... SUGGESTING THAT THE VERTICAL SHEAR IS DECREASING. ON THE OTHER HAND...THAT CONVECTION IS VERY WEAK...WITH ONLY A FEW TOPS COLDER -50C. THIS SUGGESTS THAT THE ONGOING INTERACTION WITH A NEARBY UPPER-LEVEL LOW HAS HAD AN IMPACT...AND THAT ISAAC MAY HAVE SOME SUBTROPICAL CHARACTERISTICS. SATELLITE INTENSITY ESTIMATES REMAIN 35 KT FROM ALL AGENCIES...ALTHOUGH THE DVORAK DATA-T NUMBERS REMAIN LESS THAN THAT. THE INITIAL INTENSITY REMAINS 40 KT...AND THIS COULD BE A LITTLE GENEROUS. ISAAC HAS TURNED TOWARD THE LEFT OVER THE PAST 6-12 HR. THE INITIAL MOTION IS A RATHER UNCERTAIN 305/4...AND OVER THE PAST 3-6 HR THE CYCLONE SEEMS TO BE MOVING ALMOST DUE WEST. WATER VAPOR IMAGERY INDICATES A LARGE MID/UPPER-LEVEL RIDGE BUILDING NORTH OF ISAAC IN RESPONSE TO AN UPSTREAM DEEP-LAYER TROUGH OVER THE EASTERN UNITED STATES. THIS RIDGE IS LIKELY RESPONSIBLE FOR THE LEFT TURN. THE U. S. TROUGH IS FORECAST TO MOVE EASTWARD...WITH AN ASSOCIATED SURFACE LOW FORMING OFF THE U. S. EAST COAST IN 48-72 HR. THIS DEVELOPMENT SHOULD TURN ISAAC NORTHWARD. AFTER 72 HR...THERE IS SOME MODEL DIVERGENCE. THE UKMET...NOGAPS...AND CANADIAN MOVE ISAAC NORTHEASTWARD IN ADVANCE OF THE NEW LOW...WHILE THE GFS AND ECMWF MERGE ISSAC WITH THE NEW LOW AND BRING THE CENTER NEAR EASTERN NOVA SCOTIA IN ABOUT 96 HR. FOR NOW...THE NEW FORECAST TRACK WILL RETAIN THE NORTHEASTWARD MOTION AFTER 72 HR SEEN IN THE PREVIOUS ADVISORY. THE NEW TRACK IS SHIFTED TO THE LEFT OF THE PREVIOUS FORECAST...AND AN ADDITIONAL SHIFT TO THE LEFT MAY BE NECESSARY LATER TODAY IF THE CURRENT WESTWARD TURN CONTINUES. THE INTENSITY FORECAST IS SOMEWHAT PROBLEMATIC. ISSAC SHOULD BE IN A LIGHT TO MODERATE VERTICAL SHEAR ENVIRONMENT FOR 36-48 HR...AND THE NEW FORECAST TRACK TAKES THE SYSTEM OVER SOMEWHAT WARMER SEA SURFACE TEMPERATURES THAN SEEN EARLIER. NEGATIVE FACTORS INCLUDE THE CURRENT WEAK CONVECTION AND DRY AIR SEEN IN WATER VAPOR IMAGERY NEAR AND SOUTH OF THE CYCLONE. THE GFDL RESPONDS TO THESE FACTORS BY FORECAST ISAAC TO BECOME A HURRICANE IN 72 HR. THE FSU SUPERENSEMBLE FORECASTS IT TO 48 KT IN 48 HR AND 61 KT IN 84 HR...WHILE THE SHIPS MODEL FORECASTS 44 KT IN 48 HR AND 55 KT IN 96 HR. GIVEN THE CURRENT CONVECTIVE TRENDS...THE NEW INTENSITY FORECAST WILL CHANGE LITTLE FROM THE PREVIOUS ADVISORY...CALLING FOR GRADUAL STRENGTHENING DURING THE NEXT 36 HR. AFTER THAT... INTERACTION WITH THE APPROACHING DEEP-LAYER TROUGH SHOULD STOP INTENSIFICATION AND LEAD TO EVENTUAL EXTRATROPICAL TRANSITION. FORECAST POSITIONS AND MAX WINDS INITIAL 29/0900Z 29.2N 55.6W 40 KT 12HR VT 29/1800Z 29.7N 56.4W 40 KT 24HR VT 30/0600Z 30.5N 57.7W 45 KT 36HR VT 30/1800Z 31.5N 59.0W 50 KT 48HR VT 01/0600Z 33.2N 60.0W 50 KT 72HR VT 02/0600Z 39.5N 60.0W 50 KT...BECOMING EXTRATROPICAL 96HR VT 03/0600Z 46.5N 51.0W 50 KT...EXTRATROPICAL 120HR VT 04/0600Z 49.0N 40.0W 50 KT...EXTRATROPICAL $$ FORECASTER BEVEN |
