Hurricane IRWIN

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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
 
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