ZCZC MIATCDEP1 ALL TTAA00 KNHC DDHHMM TROPICAL DEPRESSION IRWIN DISCUSSION NUMBER 41 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011 200 AM PDT SUN OCT 16 2011 A SMALL BURST OF COLD-TOPPED CONVECTION HAS REDEVELOPED OVER AND SOUTH OF THE CIRCULATION CENTER DURING THE PAST 6 HOURS....WHICH IS ENOUGH TO PRODUCE A DATA-T NUMBER OF T2.5/35 KT FROM TAFB USING THE SHEAR PATTERN TECHNIQUE. A PARTIAL ASCAT OVERPASS AT 16/0416Z CAUGHT THE WESTERN SEMICIRCLE OF IRWIN AND INDICATED A LARGE PATCH OF 25-KT WINDS EXISTED AT THAT TIME. BASED ON THE ASCAT DATA AND THE RECENT INCREASE IN DEEP CONVECTION...THE INITIAL INTENSITY IS BEING HELD AT 30 KT FOR THIS ADVISORY. THE INITIAL MOTION ESTIMATE IS 210/04 KT. IRWIN IS EXPECTED TO MOVE SLOWLY WEST-SOUTHWESTWARD AND THEN WESTWARD OVER THE NEXT 48 HOURS AROUND THE SOUTHERN PERIPHERY OF A LARGE SUBTROPICAL RIDGE THAT EXTENDS EAST-WEST ACROSS NORTHERN MEXICO AND BAJA CALIFORNIA. AFTER THAT TIME...A WEAKENED AND MORE VERTICALLY SHALLOW IRWIN IS FORECAST TO BECOME NEARLY STATIONARY OR JUST LOOP AROUND IN THE SAME GENERAL AREA AS THE LOW-LEVEL STEERING FLOW COLLAPSES. THE OFFICIAL FORECAST TRACK IS JUST AN EXTENSION OF THE PREVIOUS ADVISORY TRACK AND LIES CLOSE TO THE CONSENSUS MODEL...TVCN. ALTHOUGH IRWIN IS NOW MOVING OVER WARMER WATER AND AWAY FROM THE COLD WAKE LEFT BEHIND BY FORMER HURRICANE JOVA...WATER VAPOR IMAGERY INDICATES STRONG NORTH TO NORTHEASTERLY UPPER-LEVEL WINDS AND VERY DRY AIR READY TO AFFECT THE SYSTEM FAIRLY SOON. IN FACT... THE SHIPS INTENSITY MODEL INDICATES THE VERTICAL SHEAR WILL INCREASE TO MORE THAN 35 KT BY 36 HOURS...WHICH SHOULD INDUCE RAPID WEAKENING DESPITE THE 29C SSTS THAT THE CYCLONE WILL BE TRAVERSING. THE OFFICIAL INTENSITY FORECAST SHOWS IRWIN WEAKENING AND BECOMING A NON-CONVECTIVE REMNANT LOW PRESSURE SYSTEM WITHIN 36-48 HOURS. HOWEVER...THIS WEAKENING COULD OCCUR SOONER BASED ON THE ABUNDANCE OF VERY DRY AIR THAT LURKS JUST TO THE NORTH OF THE CYCLONE...WHICH SHOULD INHIBIT THE DEVELOPMENT OF PERSISTENT DEEP CONVECTION NEAR THE CENTER. FORECAST POSITIONS AND MAX WINDS INIT 16/0900Z 14.3N 107.5W 30 KT 35 MPH 12H 16/1800Z 14.0N 108.2W 25 KT 30 MPH 24H 17/0600Z 13.9N 108.9W 25 KT 30 MPH 36H 17/1800Z 14.0N 109.4W 25 KT 30 MPH 48H 18/0600Z 13.8N 109.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 19/0600Z 13.5N 109.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 20/0600Z 13.0N 108.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 21/0600Z...DISSIPATED $$ FORECASTER STEWART NNNN
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