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| #113419 (Received by flhurricane at: 4:43 PM 28.Sep.2006) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 5 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL092006 500 PM EDT THU SEP 28 2006 DURING THE DAY... THE STRUCTURE OF ISAAC HAS BEEN CHANGING. THE LARGER RADIUS OF MAXIMUM WINDS APPEARS TO HAVE DECREASED WITH SATELLITE IMAGES SHOWING A MORE COMPACT SYSTEM. AFTER STRUGGLING MOST OF THE DAY... MODEST CONVECTION HAS BEEN FORMING IN A CURVED BAND AROUND THE CENTER. THE CONVECTION HAS NOT BEEN AS DEEP AS TYPICALLY SEEN IN A TROPICAL CYCLONE... POSSIBLY DUE TO THE STORM MOVING OVER SLIGHTLY COOLER WATER THAT WAS STIRRED UP BY TROPICAL CYCLONES GORDON AND HELENE. MORE IMPORTANTLY... DRY AIR IS NOTED ON WATER VAPOR PICTURES NEAR THE CORE OF THE CYCLONE... WHICH ALSO MAY ACCOUNT FOR THE DESSICATED SATELLITE APPEARANCE. A FURTHER ANALYSIS OF THE 0845 UTC QUIKSCAT DATA SUGGESTS THAT WINDS OF 40-45 KT WERE PRESENT AT THAT TIME. THE INITIAL INTENSITY IS THEREFORE BUMPED UP TO 40 KT TO MATCH THIS ESTIMATE ALONG WITH THE IMPROVED CONVECTIVE STRUCTURE. THE INTENSITY FORECAST IS A BIT TRICKY DUE TO THE SOMEWHAT HYBRID NATURE OF ISAAC. FOR THE NEXT DAY OR SO... THE COLD WAKES OF GORDON AND HELENE COULD HINDER CONVECTIVE DEVELOPMENT. IN ADDITION... SOUTHERLY SHEAR FROM AN UPPER LOW DROPPING TO THE SOUTH OF THE ISAAC... ALONG WITH NEARBY DRY AIR IN THE MIDDLE TO UPPER LEVELS... SHOULD ALSO HELP TO INHIBIT DEVELOPMENT. HOWEVER BY LATE TOMORROW... VERTICAL WIND SHEAR IS FORECAST TO BECOME LIGHTER AS ISSAC BECOMES MORE UNDER THE INFLUENCE OF UPPER RIDGING TO THE NORTH. THE SHIPS MODEL ALSO DIAGNOSES AN INCREASE IN MIDDLE-LEVEL HUMIDITY BEYOND 36 HOURS AND THE STORM IS FORECAST TO MOVE OVER SLIGHTLY WARMER WATERS THAT WERE MOSTLY UNTOUCHED BY OTHER TROPICAL CYCLONES. THE INTENSITY FORECAST IS REDUCED SOMEWHAT FOR THE FIRST 24 HOURS... BUT THEN BLENDED WITH THE PREVIOUS FORECAST IN THE LATER-TERM. THIS SOLUTION IS IN REASONABLE AGREEMENT WITH A BLEND OF THE SHIPS-GFDL-FSSE (FLORIDA STATE SUPERENSEMBLE) GUIDANCE. DUE TO THE CYCLONE'S INTERACTION WITH AN UPPER LOW... THE MOTION OF ISAAC HAS SHIFTED EVEN FARTHER TO THE RIGHT... AND IS NOW 330/7. AS THE UPPER LOW MOVES SOUTHWARD... THE CIRCULATION AROUND THE UPPER LOW SHOULD HELP TO PULL THE TROPICAL CYCLONE BACK TO THE LEFT. THEREAFTER... MIDDLE LEVEL RIDGING OVER THE CENTRAL ATLANTIC SHOULD CAUSE A GENERAL NORTHWESTWARD MOTION UNTIL ABOUT 48 HOURS. A RATHER LARGE TROUGH IS FORECAST OVER THE WESTERN ATLANTIC BEYOND THAT TIME... WHICH SHOULD STEER ISAAC NORTHWARD THEN ACCELERATE IT NORTHEASTWARD. THE FORECAST TRACK HAS BEEN SHIFTED A LITTLE TO THE RIGHT EARLY ON TO ACCOUNT FOR SHORT-TERM TRENDS BUT IS SIMILAR TO THE PREVIOUS FORECAST IN THE EXTENDED PERIODS. THE OFFICAL FORECAST REMAINS JUST TO THE RIGHT OF THE MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INITIAL 28/2100Z 28.9N 54.9W 40 KT 12HR VT 29/0600Z 29.7N 55.6W 40 KT 24HR VT 29/1800Z 30.5N 57.0W 45 KT 36HR VT 30/0600Z 31.4N 58.1W 50 KT 48HR VT 30/1800Z 32.8N 59.2W 50 KT 72HR VT 01/1800Z 36.5N 60.0W 50 KT...BECOMING EXTRATROPICAL 96HR VT 02/1800Z 42.0N 56.0W 50 KT...EXTRATROPICAL 120HR VT 03/1800Z 47.0N 47.0W 50 KT...EXTRATROPICAL $$ FORECASTER BLAKE/STEWART |
