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| #113704 (Received by flhurricane at: 4:55 AM 30.Sep.2006) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 11 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL092006 500 AM AST SAT SEP 30 2006 ISAAC LOOKS MUCH MORE TROPICAL THAN IT DID 24 HR AGO...WITH CONVECTIVE TOPS COLDER THAN -60C NOT FAR NORTHWEST OF THE CENTER AND THE FIRST SIGNS OF A 30-35 NM WIDE EYE. THE CONVECTION IS NOT YET WRAPPED AROUND THE CENTER ENOUGH FOR THE DVORAK SATELLITE INTENSITY ESTIMATES TO REACH HURRICANE STRENGTH...SO ISAAC REMAINS A 60 KT TROPICAL STORM FOR THIS ADVISORY. THE STORM CURRENTLY HAS FAIR TO GOOD CIRRUS OUTFLOW IN THE NORTHERN SEMICIRCLE. THE INITIAL MOTION IS 300/6. WATER VAPOR IMAGERY SHOWS A LARGE DEEP-LAYER TROUGH OVER THE EASTERN UNITED STATES AND THE ADJACENT WESTERN ATLANTIC...WITH AN EMBEDDED SHORTWAVE TROUGH MOVING SOUTHEASTWARD THROUGH THE OHIO VALLEY. A MID/UPPER-LEVEL ANTICYCLONE IS CENTERED NEAR 36N56W WITH A SHORTWAVE RIDGE EXTENDING NORTHWARD TO NEWFOUNDLAND. THE LARGE-SCALE MODELS FORECAST THE U. S. TROUGH TO MOVE EASTWARD AND INTENSIFY OVER THE WESTERN ATLANTIC DURING THE NEXT THREE DAYS AS AN ASSOCIATED SURFACE LOW FORMS. THIS SHOULD TURN ISAAC NORTHWARD BY 36 HR AND NORTH-NORTHEASTWARD BY 72 HR IN AGREEMENT WITH THE TRACK GUIDANCE. THE NEW FORECAST TRACK IS SIMILAR TO THE PREVIOUS TRACK DURING THE FIRST 36 HR...THEN SHIFTED A LITTLE WESTWARD THEREAFTER. IT IS IN BEST AGREEMENT WITH THE CONSENSUS MODELS. DEPENDING ON HOW ISAAC INTERACTS WITH THE LOW...THERE IS A CHANCE IT COULD MOVE MORE NORTHWARD AFTER 48 HR THAN CURRENTLY FORECAST. THE LARGE-SCALE MODELS FORECAST ISAAC TO REMAIN IN A LIGHT TO MODERATE VERTICAL SHEAR ENVIRONMENT FOR ABOUT 36 HR. THE SHIPS MODELS FORECAST A 65 KT INTENSITY IN 36 HR...THE FSU SUPERENSEMBLE A 70 KT INTENSITY IN 48 HR...AND THE GFDL A 90 KT INTENSITY IN 48 HR. BASED ON THIS...THE INTENSITY FORECAST IS NUDGED UPWARD A LITTLE FROM THE PREVIOUS ADVISORY. AFTER 36-48 HR...INCREASING SHEAR AND INTERACTION WITH DEEP-LAYER TROUGH SHOULD LEAD TO WEAKENING AND EXTRATROPICAL TRANSITION. ONE DISAGREEMENT IN THE MODELS IS HOW LONG ISAAC WILL MAINTAIN ITS IDENTITY. THE GFS ABSORBS THE STORM INTO THE BAROCLINIC LOW IN ABOUT 48 HR...WHILE THE OTHER MODELS HOLD ON TO IT FOR ABOUT 72 HR. BASED ON THIS... THE TIME OF BEING ABSORBED INTO THE LOW WILL BE MOVED UP A DAY FROM THE PREVIOUS ADVISORY. FORECAST POSITIONS AND MAX WINDS INITIAL 30/0900Z 30.3N 58.0W 60 KT 12HR VT 30/1800Z 30.9N 59.0W 65 KT 24HR VT 01/0600Z 32.3N 60.4W 70 KT 36HR VT 01/1800Z 34.7N 61.0W 70 KT 48HR VT 02/0600Z 39.0N 60.2W 60 KT 72HR VT 03/0600Z 47.5N 54.5W 50 KT...INLAND EXTRATROPICAL 96HR VT 04/0600Z...ABSORBED BY EXTRATROPICAL LOW $$ FORECASTER BEVEN |
