Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
800 PM PDT THU OCT 06 2011

IRWIN HAS CONTINUED TO STRENGTHEN DURING THE PAST SEVERAL HOURS.  A
TRMM OVERPASS NEAR 2000 UTC INDICATED A WELL-ORGANIZED LOW-LEVEL
CIRCULATION CENTER...AND SINCE THAT TIME THE CLOUD TOPS HAVE BEEN
COOLING IN WHAT APPEARS TO BE A DEVELOPING EYEWALL.  SATELLITE
INTENSITY ESTIMATES FROM TAFB AND SAB ARE NOW 55 KT.  THE INITIAL
INTENSITY IS THUS INCREASED TO 55 KT...AND THIS COULD BE
CONSERVATIVE.  THE CIRRUS OUTFLOW IS GOOD TO EXCELLENT OVER THE
NORTHWESTERN SEMICIRCLE.

THE INITIAL MOTION IS AGAIN A LITTLE FASTER THAN BEFORE AT 295/9. 
IRWIN IS ON THE SOUTH SIDE OF A MID-LEVEL RIDGE THAT IS FORECAST TO
WEAKEN AS A LARGE DEEP-LAYER TROUGH OVER THE WESTERN UNITED STATES
DIGS SOUTHWARD.  THIS...IN COMBINATION WITH A LARGE AREA OF
DEVELOPING WESTERLY LOW-LEVEL FLOW ASSOCIATED WITH IRWIN AND
TROPICAL STORM JOVA TO THE EAST...IS EXPECTED TO STOP THE CURRENT
MOTION WITHIN 24-36 HOURS.  AFTER THAT...THE TRACK FORECAST
GUIDANCE IS IN GOOD AGREEMENT THAT IRWIN SHOULD MOVE
EAST-NORTHEASTWARD...ALTHOUGH THERE IS A SIGNIFICANT SPREAD IN THE
SPEED BETWEEN THE FASTER GFS AND THE SLOWER ECMWF/UKMET.  THE NEW
FORECAST TRACK IS SHIFTED TO THE WEST DURING THE FIRST 48 HOURS
BASED ON THE CURRENT POSITION AND MOTION.  AFTER THAT...IT IS
SIMILAR TO THE PREVIOUS FORECAST.  AT THIS TIME...IT APPEARS THAT
IRWIN AND JOVA ARE FAR ENOUGH APART THAT THERE WILL NOT BE ANY
DIRECT INTERACTION BETWEEN THE STORMS TO AFFECT THE MOTION.
 
IRWIN IS CURRENTLY IN A LIGHT/MODERATE VERTICAL WINDS SHEAR
ENVIRONMENT OVER SEA SURFACE TEMPERATURES OF 28C.  THESE...COMBINED
WITH THE CURRENT CONVECTIVE ORGANIZATION...ARE FAVORABLE CONDITIONS
FOR RAPID INTENSIFICATION.  THE RAPID INTENSIFICATION INDEX OF THE
SHIPS MODEL SHOWS A 40 PERCENT CHANCE OF OF 25 KT OF STRENGTHENING
DURING THE NEXT 24 HOURS...AND THE SHIPS MODEL ITSELF FORECASTS AN
80 KT INTENSITY IN 24 HOURS.  BASED ON THIS...THE INTENSITY
FORECAST DURING THE FIRST 48 HOURS IS INCREASED CONSIDERABLY FROM
THE PREVIOUS FORECAST.  AFTER THAT TIME...THE INTENSITY GUIDANCE
NOW SUGGESTS THAT INCREASING EASTERLY SHEAR WILL CAUSE IRWIN TO
WEAKEN.  BASED ON THIS...THE INTENSITY FORECAST NOW SHOWS A SLOW
WEAKENING AFTER 48 HOURS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  07/0300Z 13.4N 119.0W   55 KT  65 MPH
 12H  07/1200Z 13.7N 119.3W   65 KT  75 MPH
 24H  08/0000Z 14.1N 119.4W   80 KT  90 MPH
 36H  08/1200Z 14.4N 119.4W   85 KT 100 MPH
 48H  09/0000Z 14.5N 119.0W   85 KT 100 MPH
 72H  10/0000Z 14.5N 117.0W   80 KT  90 MPH
 96H  11/0000Z 15.5N 113.5W   75 KT  85 MPH
120H  12/0000Z 16.5N 110.5W   70 KT  80 MPH
 
$$
FORECASTER BEVEN
 
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