Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
8 AM PDT FRI AUG 26 2005

LATEST ENHANCED INFRARED IMAGERY DEPICT IRWIN AS A STRONGER TROPICAL
STORM. CONVECTION REMAINS SOUTH OF THE CENTER AS NORTHEASTERLY
SHEAR IMPACTS THE SYSTEM. THE INITIAL INTENSITY FOR IRWIN IS
INCREASED TO 45 KT BASED ON DVORAK ESTIMATES OF 3.0 FROM TAFB AND
SAB...THE UW-CIMSS OBJECTIVE DVORAK TECHNIQUE YIELDS 47 KT.

THE INTENSITY FORECAST CALLS FOR IRWIN TO REMAIN A TROPICAL STORM
OVER THE NEXT SEVERAL DAYS. IRWIN IS FORECAST TO REMAIN IN AN
ENVIRONMENT OF RELATIVELY WARM OCEAN WATERS AND MODERATE SHEAR
WHICH SHOULD ALLOW STRENGTHENING UP TO 50 KT. IN THE EXTENDED
FORECAST PERIOD...THE SHEAR INCREASES AND IRWIN CROSSES BEYOND THE
26 SST DEGREE ISOTHERM SO A DECREASE IN INTENSITY TO DEPRESSION
STRENGTH IS FORECAST. 

THE INITIAL MOTION REMAINS 280/10.  IRWIN IS EXPECTED TO REMAIN
SOUTH OF A MID-LEVEL RIDGE.  THIS RIDGE IS FORECAST TO BUILD
WESTWARD KEEPING IRWIN ON A GENERALLY WESTWARD TRACK OVER THE NEXT
SEVERAL DAYS.  THE OFFICIAL FORECAST TRACK FOLLOWS THE CONSENSUS
MODELS OUT TO 72 HRS.  BEYOND 72 HOURS...THE FORECAST TRACK IS A
BLEND OF THE FSSE AND GFS TRACKS THAT KEEP IRWIN TRACKING WESTWARD. 
THE CONSENSUS MODELS AND GFDL IN THE LATE PERIOD TRACK IRWIN ON A
NORTHWARD TRACK.  THE OFFICIAL TRACK FORECAST CALLS FOR CONTINUED
WESTWARD MOTION ALONG THE SOUTHERN EXTENT OF THE MID-LEVEL RIDGE. 
 
FORECASTER SISKO/STEWART
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      26/1500Z 17.8N 109.2W    45 KT
 12HR VT     27/0000Z 18.0N 110.7W    45 KT
 24HR VT     27/1200Z 18.1N 112.6W    50 KT
 36HR VT     28/0000Z 18.3N 114.6W    50 KT
 48HR VT     28/1200Z 18.3N 116.4W    50 KT
 72HR VT     29/1200Z 18.2N 120.1W    50 KT
 96HR VT     30/1200Z 18.0N 123.5W    40 KT
120HR VT     31/1200Z 17.5N 126.5W    30 KT...DISSIPATING
 
 
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