000 WTPZ41 KNHC 121501 TCDEP1 TROPICAL STORM IRWIN DISCUSSION NUMBER 26 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011 800 AM PDT WED OCT 12 2011 THE CENTER OF IRWIN HAS BECOME EXPOSED NEAR THE EASTERN EDGE OF A SMALL BUT PERSISTENT CONVECTIVE AREA DUE TO 15-20 KT OF EASTERLY VERTICAL WIND SHEAR. SATELLITE INTENSITY ESTIMATES ARE 35 KT FROM TAFB AND SAB...AND THE INITIAL INTENSITY REMAINS 35 KT BASED ON THESE AND A RECENT SATELLITE CONSENSUS TECHNIQUE ESTIMATE FROM CIMSS AT THE UNIVERSITY OF WISCONSIN. IRWIN HAS ACCELERATED TOWARD THE EAST-NORTHEAST DURING THE PAST SEVERAL HOURS...WITH THE INITIAL MOTION NOW 065/13. THIS IS LIKELY DUE TO INTERACTION WITH A MID/UPPER-LEVEL LOW SEEN IN WATER VAPOR IMAGERY NEAR 20N116W. THE DYNAMICAL MODELS FORECAST THIS LOW TO MOVE WESTWARD AWAY FROM IRWIN DURING THE NEXT 24 HOURS WITH A MID/UPPER-LEVEL RIDGE BUILDING NORTH OF THE TROPICAL CYCLONE. AT THE SAME TIME...IRWIN IS FORECAST TO ENCOUNTER LOW-LEVEL WESTERLY FLOW ASSOCIATED WITH A MONSOON-TYPE CIRCULATION DEVELOPING TO THE EAST. THESE FACTORS SHOULD CAUSE IRWIN TO TURN EASTWARD AND THEN SOUTHEASTWARD FROM 24-72 HOURS WITH A DECREASE IN FORWARD SPEED. AFTER 72 HOURS...THE RIDGE TO THE NORTH IS FORECAST TO BECOME THE DOMINANT STEERING INFLUENCE...CAUSING IRWIN TO TURN WESTWARD. THE MAJORITY OF THE TRACK GUIDANCE AGREES WITH THE EVOLUTION...ALTHOUGH THERE ARE SIGNIFICANT DIFFERENCES IN JUST WHEN THE VARIOUS TURNS WILL OCCUR AND HOW FAST THE CYCLONE WILL MOVE. THE NEW FORECAST TRACK IS FASTER THAN THE PREVIOUS TRACK DURING THE FIRST 72 HOURS AND BRINGS IRWIN A LITTLE FARTHER TO THE EAST. AFTER THAT IT IS SIMILAR TO THE PREVIOUS FORECAST. THE CENTER OF THE TRACK GUIDANCE ENVELOPE HAS SHIFTED NORTHWARD DURING THE FIRST 48 HOURS...AND THE NEW TRACK LIES ON THE SOUTH SIDE OF THE ENVELOPE. EASTERLY SHEAR IS LIKELY TO CONTINUE THROUGH THE FORECAST PERIOD DUE TO THE OUTFLOW FROM JOVA...TROPICAL DEPRESSION TWELVE-E...AND THE DEVELOPING MONSOON CIRCULATION. THE INTENSITY GUIDANCE SHOWS IRWIN SLOWLY WEAKENING THROUGH 72 HOURS...AND SO DOES THE NEW INTENSITY FORECAST. SOME OF THE GUIDANCE IS FORECASTING RE-INTENSIFICATION AFTER 72 HOURS. HOWEVER...THE SHIPS MODEL SHOWS INCREASING SHEAR AT THAT TIME...WHICH WOULD CAUSE IRWIN TO DEGENERATE TO A REMNANT LOW IN AGREEMENT WITH THE PREVIOUS FORECAST. THE NEW FORECAST KEEPS THE DEGENERATION SCENARIO DUE TO THE UNCERTAINTY IN THE UPPER-LEVEL LEVEL WINDS FORECAST AT 96-120 HOURS. FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 16.0N 110.7W 35 KT 40 MPH 12H 13/0000Z 16.8N 108.9W 35 KT 40 MPH 24H 13/1200Z 17.5N 106.7W 30 KT 35 MPH 36H 14/0000Z 17.6N 105.1W 30 KT 35 MPH 48H 14/1200Z 17.1N 103.9W 30 KT 35 MPH 72H 15/1200Z 16.0N 103.5W 30 KT 35 MPH 96H 16/1200Z 15.5N 105.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 17/1200Z 15.5N 108.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ FORECASTER BEVEN