000 WTPZ41 KNHC 132032 TCDEP1 TROPICAL STORM IRWIN DISCUSSION NUMBER 31 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011 200 PM PDT THU OCT 13 2011 DEEP CONVECTION HAS WANED NEAR THE CENTER OF IRWIN DURING THE PAST FEW HOURS...HOWEVER 1800 UTC DVORAK ESTIMATES FROM TAFB AND SAB SUPPORT HOLDING THE INITIAL INTENSITY AT 35 KT FOR THIS ADVISORY. THE OVERALL INTENSITY FORECAST REASONING REMAINS UNCHANGED...WITH LITTLE CHANGE EXPECTED FOR THE NEXT DAY OR SO AS IRWIN CONTINUES MOVING THROUGH A RELATIVELY DRY MID-LEVEL ENVIRONMENT DESPITE BEING SITUATED OVER VERY WARM SSTS. WHILE MOST OF THE GUIDANCE CONTINUES TO SHOW STRENGTHENING AT 48 HOURS AND BEYOND...THE OFFICIAL FORECAST WILL SHOW ONLY A SMALL INCREASE IN INTENSITY AT 2 OR 3 DAYS. A LITTLE WEAKENING IS FORECAST LATE IN THE PERIOD AS THE VERTICAL SHEAR IS EXPECTED TO INCREASE. THE NHC FORECAST IS BELOW THE INTENSITY CONSENSUS THROUGH THE PERIOD AND IS CLOSE TO THE HWRF MODEL. IRWIN HAS SLOWED AND TURNED TOWARD THE EAST OVER THE PAST FEW HOURS...AND THE INITIAL MOTION ESTIMATE IS 090/05. A MID-LEVEL RIDGE CURRENTLY CENTERED NEAR 30N 120W WILL BUILD SOUTHEASTWARD OVER THE NEXT COUPLE OF DAYS...WHICH SHOULD FORCE A IRWIN TO TURN SHARPLY TOWARD THE SOUTH AND THEN TOWARD THE SOUTH-SOUTHWEST IN THE NEXT 48 HOURS. FOR THIS CYCLE THE GUIDANCE HAS TRENDED AGAIN TOWARD A SHAPER SOUTHWARD TURN IN THE SHORT TERM WITH A SLIGHTLY FASTER FORWARD MOTION...AND SO HAS THE NHC FORECAST. BY 72 HOURS THE RIDGE BEGINS TO SHIFT NORTHWARD AND LEAVES IRWIN IN A REGION OF WEAK STEERING CURRENTS. NOT SURPRISINGLY THE SPREAD IN THE TRACK GUIDANCE INCREASES MARKEDLY AT DAYS 4 AND 5...WITH THE GFS AND UKMET SHOWING A TURN BACK TOWARD THE EAST WHILE THE ECMWF SHOWS A MUCH WEAKER IRWIN MOVING MORE QUICKLY WESTWARD. THE NHC FORECAST SHOWS A SLOW WESTWARD MOTION AT THE END OF THE PERIOD DUE TO THE LARGE UNCERTAINTY...AND HAS BEEN ADJUSTED A LITTLE TO THE SOUTH TOWARD THE NEW MULTI-MODEL CONSENSUS. GIVEN THE SMALL AREA OF TROPICAL STORM FORCE WINDS AND THE EXPECTED SHARP TURN TOWARD THE SOUTH OVER THE NEXT COUPLE OF DAYS...TROPICAL STORM WATCHES OR WARNINGS ARE NOT REQUIRED FOR THE COAST OF MEXICO AT THIS TIME. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 18.6N 106.5W 35 KT 40 MPH 12H 14/0600Z 18.4N 106.1W 35 KT 40 MPH 24H 14/1800Z 17.5N 105.7W 35 KT 40 MPH 36H 15/0600Z 16.5N 105.7W 35 KT 40 MPH 48H 15/1800Z 15.5N 106.1W 40 KT 45 MPH 72H 16/1800Z 14.5N 107.5W 40 KT 45 MPH 96H 17/1800Z 14.5N 108.5W 35 KT 40 MPH 120H 18/1800Z 14.5N 109.5W 35 KT 40 MPH $$ FORECASTER BRENNAN