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Tropical Storm IRWIN (Text)

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TROPICAL STORM IRWIN DISCUSSION NUMBER  27
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 PM PDT WED OCT 12 2011

CONVECTION ASSOCIATED WITH IRWIN HAS DECREASED THIS AFTERNOON...WITH
THE LOW-LEVEL CENTER EXPOSED TO THE EAST OF A SMALL AREA OF SHOWERS
OR THUNDERSTORMS.  AN ASCAT OVERPASS AT 1630 UTC SHOWED SEVERAL
35 KT WIND BARBS SOUTH OF THE CENTER...AND BASED ON THIS IRWIN
REMAINS A TROPICAL STORM FOR THIS ADVISORY.  THE SCATTEROMETER DATA
WAS ALSO USED TO REVISE THE INITIAL AND FORECAST WIND RADII.

THE INITIAL MOTION IS NOW 060/15.  WATER VAPOR IMAGERY INDICATES AN
UPPER-LEVEL LOW NEAR 19N117W...WHICH IS LIKELY HELPING TO STEER THE
CYCLONE EAST-NORTHEASTWARD.  AFTER 12-24 HOURS...THE DYNAMICAL
MODELS FORECAST A MID/UPPER-LEVEL RIDGE TO BUILD 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 24-48 HOURS WITH
A DECREASE IN FORWARD SPEED. AFTER THAT...THE RIDGE TO THE NORTH IS
FORECAST TO BECOME THE DOMINANT STEERING INFLUENCE...CAUSING IRWIN
TO TURN WESTWARD TO COMPLETE A LOOP OR HAIRPIN TURN.  THE NEW
FORECAST TRACK IS FASTER THAN AND NORTH OF THE PREVIOUS TRACK DURING
THE FIRST 48 HOURS BASED MAINLY ON THE INITIAL POSITION AND MOTION. 
AFTER 48 HOURS IT LIES JUST NORTH OF THE PREVIOUS FORECAST.  THE
NEW TRACK IS NEAR THE CENTER OF THE TRACK GUIDANCE ENVELOPE.
 
EASTERLY SHEAR IS LIKELY TO CONTINUE THROUGH THE FORECAST PERIOD DUE
TO THE UPPER-LEVEL ANTICYCLONE ASSOCIATED WITH JOVA...TROPICAL
DEPRESSION TWELVE-E...AND THE DEVELOPING MONSOON CIRCULATION.  THE
INTENSITY GUIDANCE SHOWS IRWIN SLOWLY WEAKENING THROUGH 48 HOURS...
AND SO DOES THE INTENSITY FORECAST.  THE INTENSITY FORECAST IS
PROBLEMATIC AFTER 48 HOURS DUE TO UNCERTAINTIES IN THE FORECAST
UPPER-LEVEL WINDS.  DUE TO THE THIS...OFFICIAL FORECAST MAINTAINS
THE PREVIOUS SCENARIO OF HAVING IRWIN DEGENERATE TO A REMNANT LOW
BY 96 HOURS.  HOWEVER... SEVERAL OF THE INTENSITY GUIDANCE MODELS
SHOW IRWIN RE-INTENSIFYING AS IT MOVES WESTWARD...AND LATER
FORECASTS MAY HAVE TO BE NUDGED IN THAT DIRECTION IF THE MODEL
TRENDS CONTINUE.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  12/2100Z 16.9N 109.4W   35 KT  40 MPH
 12H  13/0600Z 17.7N 107.7W   35 KT  40 MPH
 24H  13/1800Z 18.0N 105.6W   30 KT  35 MPH
 36H  14/0600Z 17.8N 104.5W   30 KT  35 MPH
 48H  14/1800Z 17.0N 103.8W   30 KT  35 MPH
 72H  15/1800Z 16.0N 104.5W   30 KT  35 MPH
 96H  16/1800Z 16.0N 106.5W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  17/1800Z 16.0N 109.5W   30 KT  35 MPH...POST-TROP/REMNT LOW
 
$$
FORECASTER BEVEN
 
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