Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
800 PM PDT SAT OCT 08 2011

THERE HAS BEEN NO SIGNIFICANT CONVECTION NEAR THE CENTER OF IRWIN
FOR THE PAST 12 HOURS AS EASTERLY VERTICAL WIND SHEAR CONTINUES TO
AFFECT THE CYCLONE.  THE RATHER UNCERTAIN INITIAL INTENSITY OF 50
KT IS BASED ON A BLEND OF SATELLITE INTENSITY ESTIMATES FROM SAB
AND TAFB...AND IT IS POSSIBLE THIS COULD BE GENEROUS.

THE INITIAL MOTION IS NOW 050/3.  WATER VAPOR IMAGERY SHOWS A LARGE
MID/UPPER-LEVEL TROUGH OVER THE WESTERN UNITED STATES AND
NORTHWESTERN MEXICO.  THE DYNAMICAL MODELS FORECAST THIS TROUGH TO
MOVE NORTHEASTWARD DURING THE NEXT 72 HOURS AS A SECOND TROUGH
MOVES SOUTHEASTWARD INTO NORTHWESTERN MEXICO.  THIS TROUGH IS
FORECAST TO BECOME A CUT-OFF LOW OVER THE PACIFIC WEST OF MEXICO BY
120 HOURS.  THIS EVOLUTION IS FORECAST TO STEER IRWIN EAST-
NORTHEASTWARD TO EASTWARD DURING THE NEXT 72 HOURS.  AFTER THAT
TIME...THERE IS SIGNIFICANT SPREAD IN THE MODEL GUIDANCE.  THE GFS
SHOW IRWIN RESPONDING STRONGLY TO THE NEW TROUGH AND TURNING
NORTHWARD SOUTH OF BAJA CALIFORNIA.  THE GFDL SHOWS A CONTINUED
EASTWARD MOTION...WITH THE REMAINDER OF THE DYNAMICAL MODELS SPREAD
BETWEEN THESE EXTREMES.  OVERALL...THE GUIDANCE IS A LITTLE FASTER
AND A LITTLE FARTHER TO THE EAST THAN ON THE PREVIOUS ADVISORY. 
THE NEW FORECAST TRACK CALLS FOR A GRADUAL TURN TOWARD THE
NORTH-NORTHEAST AFTER 72 HOURS IN BEST AGREEMENT WITH THE CANADIAN
MODEL.  THE NEW TRACK LIES TO THE LEFT OF THE CONSENSUS MODEL.

THE INTENSITY FORECAST IS BASED ON THE PREMISE THAT CONVECTION
RE-DEVELOPS NEAR THE CENTER OF IRWIN AND THAT THE CYCLONE DOES NOT
DECAY TO A REMNANT LOW.  THE LATEST DYNAMICAL MODEL RUNS SUGGEST
THAT MODERATE EASTERLY SHEAR MAY CONTINUE FOR AS LONG AS 72 HOURS
BEFORE CONDITIONS BECOME MORE FAVORABLE FOR STRENGTHENING. 
INDEED...THE GFDL AND HWRF MODELS SHOW LITTLE CHANGE IN STRENGTH
FOR 72 HOURS OR SO...FOLLOWED BY INTENSIFICATION.  THE INTENSITY
FORECAST IS SIGNIFICANTLY CHANGED FROM THAT OF THE PREVIOUS
ADVISORY...CALLING FOR IRWIN TO WEAKEN A LITTLE MORE BEFORE
MAINTAINING AN INTENSITY NEAR 45 KT THROUGH 72 HOURS.  AFTER THAT
TIME...WARMER SEA SURFACE TEMPERATURES AND DECREASED SHEAR MAY
ALLOW SOME INTENSIFICATION.  THE NEW INTENSITY FORECAST SHOWS
THIS...BUT IS NOT AS STRONG AS THE GFDL AND HWRF MODELS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  09/0300Z 14.7N 120.3W   50 KT  60 MPH
 12H  09/1200Z 15.0N 119.7W   45 KT  50 MPH
 24H  10/0000Z 15.3N 118.5W   45 KT  50 MPH
 36H  10/1200Z 15.5N 117.3W   45 KT  50 MPH
 48H  11/0000Z 15.6N 115.8W   45 KT  50 MPH
 72H  12/0000Z 15.5N 113.0W   50 KT  60 MPH
 96H  13/0000Z 17.0N 109.0W   55 KT  65 MPH
120H  14/0000Z 20.0N 107.5W   60 KT  70 MPH
 
$$
FORECASTER BEVEN
 
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