Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER  26
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
800 AM PDT WED OCT 12 2011

THE CENTER OF IRWIN HAS BECOME EXPOSED NEAR THE EASTERN EDGE OF A
SMALL BUT PERSISTENT CONVECTIVE AREA DUE TO 15-20 KT OF EASTERLY
VERTICAL WIND SHEAR.  SATELLITE INTENSITY ESTIMATES ARE 35 KT FROM
TAFB AND SAB...AND THE INITIAL INTENSITY REMAINS 35 KT BASED ON
THESE AND A RECENT SATELLITE CONSENSUS TECHNIQUE ESTIMATE FROM
CIMSS AT THE UNIVERSITY OF WISCONSIN.

IRWIN HAS ACCELERATED TOWARD THE EAST-NORTHEAST DURING THE PAST
SEVERAL HOURS...WITH THE INITIAL MOTION NOW 065/13.  THIS IS LIKELY
DUE TO INTERACTION WITH A MID/UPPER-LEVEL LOW SEEN IN WATER VAPOR
IMAGERY NEAR 20N116W.  THE DYNAMICAL MODELS FORECAST THIS LOW TO
MOVE WESTWARD AWAY FROM IRWIN DURING THE NEXT 24 HOURS WITH A
MID/UPPER-LEVEL RIDGE BUILDING NORTH OF THE TROPICAL CYCLONE.  AT
THE SAME TIME...IRWIN IS FORECAST TO ENCOUNTER LOW-LEVEL WESTERLY
FLOW ASSOCIATED WITH A MONSOON-TYPE CIRCULATION DEVELOPING TO THE
EAST.  THESE FACTORS SHOULD CAUSE IRWIN TO TURN EASTWARD AND THEN
SOUTHEASTWARD FROM 24-72 HOURS WITH A DECREASE IN FORWARD SPEED. 
AFTER 72 HOURS...THE RIDGE TO THE NORTH IS FORECAST TO BECOME THE
DOMINANT STEERING INFLUENCE...CAUSING IRWIN TO TURN WESTWARD.  THE
MAJORITY OF THE TRACK GUIDANCE AGREES WITH THE EVOLUTION...ALTHOUGH
THERE ARE SIGNIFICANT DIFFERENCES IN JUST WHEN THE VARIOUS TURNS
WILL OCCUR AND HOW FAST THE CYCLONE WILL MOVE.  THE NEW FORECAST
TRACK IS FASTER THAN THE PREVIOUS TRACK DURING THE FIRST 72 HOURS
AND BRINGS IRWIN A LITTLE FARTHER TO THE EAST.  AFTER THAT IT IS
SIMILAR TO THE PREVIOUS FORECAST.  THE CENTER OF THE TRACK GUIDANCE
ENVELOPE HAS SHIFTED NORTHWARD DURING THE FIRST 48 HOURS...AND THE
NEW TRACK LIES ON THE SOUTH SIDE OF THE ENVELOPE.

EASTERLY SHEAR IS LIKELY TO CONTINUE THROUGH THE FORECAST PERIOD DUE
TO THE OUTFLOW FROM JOVA...TROPICAL DEPRESSION TWELVE-E...AND THE
DEVELOPING MONSOON CIRCULATION.  THE INTENSITY GUIDANCE SHOWS IRWIN
SLOWLY WEAKENING THROUGH 72 HOURS...AND SO DOES THE NEW INTENSITY
FORECAST.  SOME OF THE GUIDANCE IS FORECASTING RE-INTENSIFICATION
AFTER 72 HOURS.  HOWEVER...THE SHIPS MODEL SHOWS INCREASING SHEAR
AT THAT TIME...WHICH WOULD CAUSE IRWIN TO DEGENERATE TO A REMNANT
LOW IN AGREEMENT WITH THE PREVIOUS FORECAST.  THE NEW FORECAST
KEEPS THE DEGENERATION SCENARIO DUE TO THE UNCERTAINTY IN THE
UPPER-LEVEL LEVEL WINDS FORECAST AT 96-120 HOURS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  12/1500Z 16.0N 110.7W   35 KT  40 MPH
 12H  13/0000Z 16.8N 108.9W   35 KT  40 MPH
 24H  13/1200Z 17.5N 106.7W   30 KT  35 MPH
 36H  14/0000Z 17.6N 105.1W   30 KT  35 MPH
 48H  14/1200Z 17.1N 103.9W   30 KT  35 MPH
 72H  15/1200Z 16.0N 103.5W   30 KT  35 MPH
 96H  16/1200Z 15.5N 105.5W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  17/1200Z 15.5N 108.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 
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FORECASTER BEVEN
 
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