Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER  11
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 PM PDT SAT OCT 08 2011
 
THE VERTICAL SHEAR CONTINUES TO ADVERSELY AFFECT IRWIN AND IT
APPEARS THAT THE CYCLONE HAS DROPPED BELOW HURRICANE STRENGTH.  AN
INITIAL INTENSITY OF 60 KT IS SUPPORTED BY A BLEND OF THE TAFB AND
SAB SUBJECTIVE DVORAK ESTIMATES...THE CIMSS ADVANCED DVORAK...AND
THE CIMSS AMSU ANALYSIS.  A 1242Z AMSU PASS WAS ALSO USEFUL FOR
DIAGNOSING THE WIND RADII FROM THE CIRA SIZE ANALYSIS.
 
THE TROPICAL STORM IS CURRENTLY EMBEDDED IN A VERY WEAK STEERING
FLOW AND INITIAL MOTION IS SET AT 360 DEGREES AT 1 KT. THE CYCLONE
SHOULD START MOVING OFF TOWARD THE EAST WITH INCREASING FORWARD
SPEED OVER THE NEXT FEW DAYS AS IT RESPONDS TO A SHORT-WAVE TROUGH
DIGGING DOWN OVER THE SOUTHWESTERN UNITED STATES AND NORTHEASTERN
MEXICO. THE INFLUENCE ON THE STEERING BY NEARBY TROPICAL STORM JOVA
IS MINIMAL CURRENTLY...THOUGH MORE OF THE MODEL GUIDANCE THIS
AFTERNOON HAS SUGGESTED SOME FUIJIWHARA INTERACTION AFTER DAY TWO.
THIS HAS THE EFFECT OF A SLIGHTLY SOUTH OF DUE EAST TRACK FOR IRWIN
ON DAY THREE DUE TO A MUTUAL COUNTER-CLOCKWISE ROTATION OF THE TWO
VORTICES. THE FORECAST TRACK IS SIMILAR TO THE MULTI-MODEL
CONSENSUS AND SLIGHTLY FASTER THAN THE PREVIOUS ADVISORY.
 
THE INTENSITY FORECAST OF IRWIN CONTINUES TO BE QUITE DIFFICULT. THE
EASY PART IS THAT THE SEA SURFACE TEMPERATURES UNDER THE TRACK OF
THE CYCLONE SHOULD REMAIN A VERY WARM 28C. ALSO...THE VERTICAL
SHEAR CURRENTLY PLAGUING THE CYCLONE SHOULD SOMEWHAT ABATE DURING
THE NEXT DAY OR TWO...ALLOWING A MODERATE REBOUND IN THE INTENSITY
TO OCCUR. HOWEVER...BY DAY THREE THE SHEAR MAY AGAIN INCREASE AT
THE SAME TIME THAT THE ATMOSPHERE BECOMES MORE STABLE TO DEEP
CONVECTION. THE GFDL MODEL KEEPS IRWIN A TROPICAL STORM...WHILE THE
HWRF BRINGS IT BACK ALL THE WAY TO A MAJOR HURRICANE. WHILE THE
LGEM/SHIPS MODELS WERE RUN THIS AFTERNOON...BECAUSE OF TECHNICAL
PROBLEMS...THEIR OUTPUT IS BASED UPON THE BAM-MEDIUM TAKING THE
CYCLONE TOO FAR NORTH AND CANNOT BE UTILIZED DIRECTLY. THEREFORE
THE INTENSITY FORECAST IS SLIGHTLY LOWER THAN THE PREVIOUS ADVISORY
DUE TO THE WEAKENED INITIAL STATE...BUT OTHERWISE HAS LITTLE
CHANGE.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  08/2100Z 14.4N 120.9W   60 KT  70 MPH
 12H  09/0600Z 14.7N 120.6W   60 KT  70 MPH
 24H  09/1800Z 14.9N 119.5W   65 KT  75 MPH
 36H  10/0600Z 15.1N 118.4W   70 KT  80 MPH
 48H  10/1800Z 15.2N 116.6W   65 KT  75 MPH
 72H  11/1800Z 15.0N 114.0W   60 KT  70 MPH
 96H  12/1800Z 16.0N 110.0W   55 KT  65 MPH
120H  13/1800Z 19.0N 108.0W   50 KT  60 MPH
 
$$
FORECASTER LANDSEA/STEWART
 
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