000 WTPZ41 KNHC 070839 TCDEP1 HURRICANE IRWIN DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011 200 AM PDT FRI OCT 07 2011 OVER THE PAST COUPLE OF HOURS...AN EYE HAS BECOME EVIDENT IN INFRARED SATELLITE IMAGERY AND IS GRADUALLY WARMING. THERE ALSO WAS A LULL IN THE DEEP CONVECTION...BUT NEW CONVECTION IS DEVELOPING IN THE EYEWALL. DVORAK ESTIMATES FROM TAFB AND SAB WERE 65 KT AT 0600 UTC...AND THE LATEST UW-CIMSS ADT ESTIMATE IS 70-75 KT. BASED ON THESE DATA AND THE IMPROVED CONVECTION SINCE THE SYNOPTIC HOUR...IRWIN IS NOW A HURRICANE WITH AN INITIAL INTENSITY OF 70 KT. IRWIN HAS NOT SLOWED DOWN AS HAS BEEN FORECAST...AND THE INITIAL MOTION IS 300/9 KT. ALL OF THE TRACK GUIDANCE INDICATES THAT THE HURRICANE SHOULD VIRTUALLY STOP ON A DIME DURING THE NEXT 24 HOURS AS THE STEERING CURRENTS AROUND THE CYCLONE COLLAPSE. A VERY SLOW EASTWARD TURN IS EXPECTED BY 36 HOURS AS IRWIN BARELY FEELS THE INFLUENCE OF A MID-LATITUDE TROUGH OVER THE SOUTHWESTERN UNITED STATES. ALTHOUGH THE TROUGH IS FORECAST TO REMAIN BROAD...THE WESTERLIES ARE EXPECTED TO MOVE FARTHER SOUTH AND BEGIN TO ACCELERATE IRWIN EASTWARD BY DAYS 4 AND 5. THE TRACK MODELS AGREE ON THE GENERAL FORECAST PATH BUT DIFFER ON THE FORECAST SPEED... ESPECIALLY BY DAYS 4 AND 5. THE NEW OFFICIAL FORECAST IS A LITTLE SOUTH OF THE PREVIOUS FORECAST AT THAT TIME...LEANING IN THE DIRECTION OF THE ECMWF AND THE MODEL CONSENSUS. IRWIN HAS BEEN GOING THROUGH A PERIOD OF RAPID INTENSIFICATION WITH A CHANGE OF 50 KT OVER THE PAST 24 HOURS. AN 0145 UTC WINDSAT PASS INDICATED THAT IRWIN HAS A STRUCTURE THAT IS CONDUCIVE FOR RAPID INTENSIFICATION...AND THE HURRICANE IS OVER SSTS OF ABOUT 28C...SUFFICIENTLY WARM ENOUGH TO SUPPORT ADDITIONAL STRENGTHENING. IN FACT...THE RI INDEX IN THE NEWEST SHIPS GUIDANCE INDICATES THAT THERE IS A 3 IN 10 CHANCE OF ANOTHER PERIOD OF RI DURING THE NEXT 24 HOURS. THE OFFICIAL FORECAST IS THEREFORE HIGHER THAN MUCH OF THE GUIDANCE FOR THE FIRST 24 HOURS BEFORE LEVELING OFF NEAR THE SHIPS OUTPUT FROM 36-72 HOURS. ONE ARGUMENT AGAINST POSSIBLE RI IS THE FACT THAT THE THERMOCLINE IS RELATIVELY SHALLOW NEAR IRWIN...AND THE EXPECTED SLOW MOTION OVER THE NEXT 48 HOURS COULD UPWELL COLDER WATER AND LIMIT INTENSIFICATION. GRADUAL WEAKENING IS INDICATED DURING DAYS 3 THROUGH 5 DUE TO AN INCREASE IN VERTICAL SHEAR...BUT THE OFFICIAL FORECAST IS STILL HIGHER THAN THE STATISTICAL GUIDANCE DURING THAT PERIOD. FORECAST POSITIONS AND MAX WINDS INIT 07/0900Z 13.7N 119.8W 70 KT 80 MPH 12H 07/1800Z 14.1N 120.1W 80 KT 90 MPH 24H 08/0600Z 14.4N 120.2W 90 KT 105 MPH 36H 08/1800Z 14.4N 119.8W 90 KT 105 MPH 48H 09/0600Z 14.4N 119.0W 85 KT 100 MPH 72H 10/0600Z 15.0N 116.5W 80 KT 90 MPH 96H 11/0600Z 15.5N 113.0W 75 KT 85 MPH 120H 12/0600Z 16.0N 109.5W 70 KT 80 MPH $$ FORECASTER BERG