Hurricane IRWIN

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HURRICANE IRWIN DISCUSSION NUMBER   8
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
800 PM PDT FRI OCT 07 2011

IRWIN HAS A WELL-DEFINED CENTRAL DENSE OVERCAST THIS EVENING...WITH
CLOUD TOPS COLDER THAN -80C TO THE SOUTHEAST OF THE CENTER. WHILE
THE EYE SEEN EARLIER IN SATELLITE IMAGES IS NOT CURRENTLY
APPARENT...EARLIER MICROWAVE IMAGES SUGGEST IT IS STILL PRESENT
UNDER THE OVERCAST. THE SATELLITE INTENSITY ESTIMATES FROM TAFB AND
SAB HAVE NOT CHANGED...SO THE INITIAL INTENSITY REMAINS 80 KT.
ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN SHOW ABOUT 15 KT
OF EASTERLY VERTICAL WIND SHEAR OVER IRWIN...AND THIS IS CAUSING
THE OUTFLOW TO THE EAST TO BE RESTRICTED.

IRWIN CONTINUES TO SLIDE FARTHER WEST-NORTHWESTWARD THAN FORECAST...
WITH THE INITIAL MOTION 295/4. THE TRACK GUIDANCE IS IN GOOD
AGREEMENT THAT THE FORWARD MOTION SHOULD COME TO A HALT LATER
TONIGHT AS A LARGE MID/UPPER-LEVEL TROUGH TO THE NORTH INCREASES
ITS INFLUENCE ON IRWIN. THIS TROUGH IS FORECAST TO CAUSE THE
HURRICANE TO TURN TOWARD THE EAST-NORTHEAST BY 36 HOURS...AND
TOWARD THE EAST AFTER 36 HOURS. THE TRACK GUIDANCE IS IN GOOD
AGREEMENT ON THE DIRECTION OF MOTION...BUT THERE IS SOME SPREAD IN
THE FORWARD SPEED BETWEEN THE FASTER ECMWF AND THE SLOWER
GFS/NOGAPS MODELS. OVERALL...THE NEW GUIDANCE IS A LITTLE SLOWER
THAN EARLIER...AND THUS THE NEW FORECAST TRACK IS A LITTLE SLOWER
THAN THE PREVIOUS TRACK. THE NEW TRACK LIES NEAR THE CENTER OF THE
TRACK GUIDANCE ENVELOPE.

THE DYNAMICAL MODELS FORECAST IRWIN TO ENCOUNTER INCREASED SHEAR AS
IT MAKES ITS EASTWARD TURN IN 24-36 HOURS.  THE INTENSITY FORECAST
SHOWS SOME SLOW STRENGTHENING BEFORE THAT TIME...AND SLOW WEAKENING
AFTER THAT TIME.  THE NEW INTENSITY FORECAST IS SIMILAR TO THE
SHIPS AND LGEM MODELS FOR THE FIRST 36 HOURS...AND A LITTLE
STRONGER THAN THOSE MODELS AFTER THAT TIME.  AS MENTIONED IN THE
PREVIOUS DISCUSSION...THERE IS A POSSIBILITY THAT CONDITIONS COULD
BECOME MORE FAVORABLE FOR STRENGTHENING AT ABOUT 120 HOURS. 
HOWEVER...THE EVIDENCE OF THAT OCCURRING NEEDS TO BE STRONGER
BEFORE THE FORECAST CAN SHOW STRENGTHENING AT THAT TIME.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  08/0300Z 14.1N 121.3W   80 KT  90 MPH
 12H  08/1200Z 14.4N 121.4W   80 KT  90 MPH
 24H  09/0000Z 14.7N 121.1W   85 KT 100 MPH
 36H  09/1200Z 14.9N 120.5W   85 KT 100 MPH
 48H  10/0000Z 15.0N 119.5W   80 KT  90 MPH
 72H  11/0000Z 15.0N 117.0W   70 KT  80 MPH
 96H  12/0000Z 15.0N 113.5W   65 KT  75 MPH
120H  13/0000Z 16.5N 109.0W   60 KT  70 MPH
 
$$
FORECASTER BEVEN
 
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