000 WTPZ41 KNHC 080854 TCDEP1 HURRICANE IRWIN DISCUSSION NUMBER 9 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011 200 AM PDT SAT OCT 08 2011 IT APPEARS THAT THERE IS A LITTLE BIT OF EASTERLY SHEAR AFFECTING IRWIN...AND THE LOW-LEVEL CENTER MAY NOW BE LOCATED CLOSER TO THE EASTERN EDGE OF THE CENTRAL DENSE OVERCAST. SUBJECTIVE AND OBJECTIVE DVORAK CURRENT INTENSITY ESTIMATES RANGE FROM 77 KT TO 90 KT...AND THE INTENSITY IS BEING KEPT AT 80 KT SINCE THERE HAS NOT BEEN AN INCREASE IN ORGANIZATION SINCE THE LAST ADVISORY. IRWIN IS MOVING SLOWLY NORTHWESTWARD...OR 315 DEGREES AT 4 KT. THE CYCLONE IS TRAPPED WITHIN A WEAK STEERING REGIME BETWEEN TROPICAL STORM JOVA TO ITS EAST...A LARGE SUBTROPICAL RIDGE TO ITS NORTHWEST...AND A MID-LATITUDE TROUGH TO THE NORTHEAST. THIS WEAK STEERING IS EXPECTED TO CONTINUE FOR ANOTHER 36 TO 48 HOURS UNTIL A MID-LATITUDE SHORTWAVE TROUGH DIGS SOUTHWARD OFF THE BAJA CALIFORNIA COAST. IRWIN SHOULD THEN ACCELERATE TOWARD THE EAST-NORTHEAST FROM 48 TO 96 HOURS. THE GLOBAL MODELS ARE SUGGESTING THAT THE SHORTWAVE TROUGH WILL CUTOFF IN 3 TO 4 DAYS...AND THIS WOULD LIKELY CAUSE IRWIN TO BEGIN TURNING NORTHEASTWARD AROUND THE SOUTHEASTERN SIDE OF THE CUT-OFF LOW BY DAY 5. THE TRACK GUIDANCE IS IN GOOD AGREEMENT ON THIS SCENARIO...AND ONLY A SLIGHT NORTHWARD SHIFT FROM THE PREVIOUS FORECAST WAS REQUIRED ON DAY 5. THE SHIPS DIAGNOSTICS ARE INDICATING THAT THE ENVIRONMENT IN THE VICINITY OF IRWIN MAY BE TOO STABLE AND TOO WARM IN THE UPPER LEVELS OF THE ATMOSPHERE TO SUPPORT SIGNIFICANT INTENSIFICATION. THEREFORE...LITTLE CHANGE IN INTENSITY IS INDICATED IN THE OFFICIAL FORECAST THROUGH 36 HOURS. GRADUAL WEAKENING IS FORECAST TO COMMENCE AFTER 36 HOURS DUE TO THE AFOREMENTIONED ENVIRONMENTAL CONDITIONS...DECREASING SEA SURFACE TEMPERATURES...AND INCREASING VERTICAL SHEAR ON DAYS 3 AND 4 ONCE THE SHORTWAVE TROUGH CUTS OFF. THIS FORECAST IS CLOSER TO THE STATISTICAL GUIDANCE THAN THE MORE AGGRESSIVE GFDL AND HWRF MODELS. FORECAST POSITIONS AND MAX WINDS INIT 08/0900Z 14.4N 121.1W 80 KT 90 MPH 12H 08/1800Z 14.8N 120.9W 80 KT 90 MPH 24H 09/0600Z 15.1N 120.5W 80 KT 90 MPH 36H 09/1800Z 15.3N 119.8W 80 KT 90 MPH 48H 10/0600Z 15.4N 118.7W 75 KT 85 MPH 72H 11/0600Z 15.5N 116.0W 70 KT 80 MPH 96H 12/0600Z 16.0N 112.0W 65 KT 75 MPH 120H 13/0600Z 18.5N 108.0W 60 KT 70 MPH $$ FORECASTER BERG