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| #113811 (Received by flhurricane at: 11:11 PM 30.Sep.2006) TCDAT4 HURRICANE ISAAC DISCUSSION NUMBER 14 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL092006 1100 PM EDT SAT SEP 30 2006 LATEST GOES IMAGERY SHOWS THAT THE EYE OF ISAAC HAS INTERMITTENTLY APPEARING AND DISAPPEARING THROUGHOUT THE EVENING. GIVEN A LACK OF SIGNIFICANT CHANGE IN THE SATELLITE PRESENTATION...THE INITIAL INTENSITY IS HELD AT 70 KT FOR THIS ADVISORY...BASED ON A BLEND OF DVORAK INTENSITY ESTIMATES BETWEEN 65 AND 77 KT. THE INTENSITY FORECAST CONTINUES TO BE PROBLEMATIC GIVEN OFFSETTING FACTORS IN THE SHORT-TERM...AND A COMPLEX INTERACTION WITH A MID-LATITUDE TROUGH THEREAFTER. ISAAC HAS ABOUT 12-24 HOURS OVER WATERS OF AT LEAST 26C. HOWEVER...WATER VAPOR IMAGERY SHOWS THAT ISAAC IS EXPERIENCING INCREASING SOUTHERLY SHEAR. BEYOND 24 HOURS...THE HURRICANE WILL BEGIN TO INTERACT WITH A LARGE MID-LATITUDE TROUGH MOVING OFF THE EAST COAST OF THE UNITED STATES. THE GLOBAL MODELS DISAGREE ON WHETHER ISAAC WILL BE ABLE TO UTILIZE THE AVAILABLE BAROCLINIC ENERGY TO STRENGTHEN DURING EXTRATROPICAL TRANSITION... OR INSTEAD BE ABSORBED BY A MORE DOMINANT EXTRATROPICAL LOW WHICH COULD FORM TO ITS WEST. SHOULD ISAAC MERGE WITH A SECOND LOW...IT WOULD PROBABLY PRODUCE A BROADER AND WEAKER PRESSURE GRADIENT. CONVERSELY...IF ISAAC REMAINS THE DOMINANT SYSTEM...IT COULD RE-INTENSIFY AS AN EXTRATROPICAL CYCLONE...IN A SIMILAR MANNER TO HELENE. THE OFFICIAL FORECAST ESSENTIALLY SPLITS THE DIFFERENCE BETWEEN THESE TWO SCENARIOS BY SHOWING SLOW WEAKENING THROUGH THE FIRST 48 HOURS. THE NEW OFFICIAL FORECAST NOW SHOWS SLIGHT WEAKENING IN THE SHORT-TERM AND LITTLE CHANGE DURING EXTRATROPICAL TRANSITION. ISAAC IS MOVING JUST A LITTLE TO THE RIGHT...OR EAST...OF THE PREVIOUS ADVISORY WITH AN INITIAL MOTION ESTIMATE OF 330/09. GLOBAL MODELS REMAIN IN GENERAL AGREEMENT ON A GRADUAL NORTHWARD TURN THROUGH 24 HOURS...FOLLOWED BY ACCELERATION TO THE NORTHEAST AS THE CYCLONE GETS CAUGHT UP IN SOUTHWESTERLY FLOW AHEAD OF THE MID-LATITUDE TROUGH. THE ECMWF...GFS...AND GFS ENSEMBLE MEAN ARE ALONG THE LEFT SIDE OF THE MODEL SUITE AND SHOW THE CYCLONE MOVING OVER OR NEAR NEWFOUNDLAND. CONVERSELY...THE GFDL AND GFDN ARE MUCH FARTHER EAST. REMAINING IN BETWEEN THOSE EXTREMES AND CLOSE TO THE MODEL CONSENSUS...THE OFFICIAL FORECAST IS NUDGED A LITTLE TO THE RIGHT DURING THE FIRST 36 HOURS...AND IS ESSENTIALLY AN UPDATE OF THE PREVIOUS FORECAST THEREAFTER. FORECAST POSITIONS AND MAX WINDS INITIAL 01/0300Z 32.5N 59.9W 70 KT 12HR VT 01/1200Z 34.0N 60.5W 70 KT 24HR VT 02/0000Z 37.2N 60.2W 65 KT 36HR VT 02/1200Z 42.0N 57.9W 60 KT...BECOMING EXTRATROPICAL 48HR VT 03/0000Z 46.8N 53.5W 60 KT...EXTRATROPICAL 72HR VT 04/0000Z 53.0N 43.0W 50 KT...EXTRATROPICAL 96HR VT 05/0000Z 56.0N 30.0W 45 KT...EXTRATROPICAL 120HR VT 06/0000Z...ABSORBED $$ FORECASTER RHOME/KNABB |
