Hurricane IRWIN

ZCZC MIATCDEP1 ALL
TTAA00 KNHC DDHHMM
 
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
 
NNNN