ZCZC MIATCDEP4 ALL TTAA00 KNHC DDHHMM TROPICAL STORM IRWIN DISCUSSION NUMBER 8 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 8 AM PDT SAT AUG 27 2005 A SMALL CLUSTER OF DEEP CONVECTION HAS PERSISTED JUST WEST OF THE EXPOSED LOW-LEVEL CIRCULATION CENTER...WHICH IS JUST ENOUGH TO KEEP IRWIN A 35-KT TROPICAL STORM. MODERATE EAST-SOUTHEASTERLY UPPER-LEVEL SHEAR IS EXPECTED TO PERSIST FOR THE NEXT 96 HOURS... WITH SOME RELAXATION OF THE SHEAR EXPECTED AFTER THAT. GIVEN THAT IRWIN IS FORECAST TO REMAIN OVER 26-27C SSTS THROUGHOUT THE FORECAST PERIOD...THERE MAY BE ENOUGH INTERMITTENT BURSTS OF DEEP CONVECTION NEAR THE CENTER TO KEEP THIS SYSTEM GOING AS A MINIMAL TROPICAL CYCLONE. THIS IS CONSISTENT WITH THE SHIPS MODEL...WHICH ONLY WEAKENS IRWIN TO 25 KT IN 120 HOURS BENEATH 12 KT OF SHEAR. THE INITIAL MOTION REMAINS AT 270/9. THE GENERAL MOTION SHOULD BE WESTWARD AS IRWIN IS STEERED BY THE DEEP-LAYER EASTERLY FLOW ON THE SOUTH SIDE OF A STRONG MID-LEVEL SUBTROPICAL RIDGE THAT EXTENDS FROM MEXICO WESTWARD TO THE NORTH OF IRWIN. THIS MOTION IS SUPPORTED BY THE CONSENSUS OF THE NHC MODEL GUIDANCE. HOWEVER... IRWIN IS EXPECTED TO REMAIN OVER OR VERY NEAR SSTS WARM ENOUGH TO SUPPORT THE OCCASIONAL DEVELOPMENT OF DEEP CONVECTION. THIS INTERMITTENT STRENGTHENING OF THE VERTICAL CIRCULATION IN CONJUNCTION WITH SOUTHEAST TO EAST-SOUTHEASTERLY UPPER-LEVEL SHEAR MAY RESULT IN OCCASIONAL WOBBLES TO THE RIGHT...OR NORTH...OF THE MEAN WESTERLY FORECAST TRACK INDICATED IN THE OFFICIAL ADVISORY. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 27/1500Z 17.4N 112.6W 35 KT 12HR VT 28/0000Z 17.4N 114.0W 35 KT 24HR VT 28/1200Z 17.5N 115.9W 35 KT 36HR VT 29/0000Z 17.6N 117.7W 35 KT 48HR VT 29/1200Z 17.7N 119.1W 35 KT 72HR VT 30/1200Z 17.6N 122.0W 30 KT 96HR VT 31/1200Z 17.5N 126.0W 25 KT 120HR VT 01/1200Z 17.5N 130.0W 25 KT $$ NNNN
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Page last modified: Saturday, 27-Aug-2005 14:55:14 UTC