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Tropical Storm ISIS (Text)

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TROPICAL STORM ISIS DISCUSSION NUMBER  20
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
8 PM PDT SUN SEP 12 2004

CONVECTION CONTINUES TO BURST ON THE PERIPHERY OF ISIS BUT AWAY FROM
THE LOW LEVEL CIRCULATION CENTER.  ISIS REMAINS SIGNIFICANTLY
SHEARED BUT IS STILL A 45 KT TROPICAL STORM...AS CLASSIFIED BY TAFB
AND AIR FORCE DVORAK ESTIMATES.  THE INITIAL MOTION HAS NOT CHANGED
AND REMAINS AT 270/8.

THE TRACK FORECAST IS ON TOP OF THE PREVIOUS TRACK FOR THE FIRST 24
HOURS.  THEREAFTER...MOST GUIDANCE ARE BUILDING IN A STRONG RIDGE
AHEAD OF THE FORECAST TRACK.  THIS WOULD WORK TO SLOW THE FORWARD
MOTION OF ISIS AND EVENTUALLY STEER THE STORM TO THE NORTHWEST OR
NORTH WHEN A TROUGH IS FORECAST TO BREAK DOWN THE RIDGE DURING DAYS
4 AND 5. ALL GUIDANCE EXCEPT UKMET SHOWS A NORTHWARD COMPONENT OF
MOTION AT THE 120 HOUR FORECAST PERIOD.  SLIGHT ADJUSTMENTS TO THE
FORECAST TRACK TOWARD THE NORTH ARE MADE TO ACCOUNT FOR THIS
SCENARIO...WHICH INCLUDES A CONTINUING TREND TOWARD A SLOWER
FORWARD SPEED.  THIS IS NOT A HIGH CONFIDENCE FORECAST BEYOND 72
HOURS.       

ALL INTENSITY GUIDANCE CONTINUES TO SUGGEST A GRADUAL STRENGTHENING
OF ISIS.  THIS IS CONTINGENT UPON A WEAKENING OF THE PERSISTENT
EASTERLY SHEAR THAT HAS BEEN AFFECTING ISIS SINCE ITS GENESIS.  THE
SHEAR PREDICTOR IN THE SHIPS MODEL SUGGESTS VERY LOW SHEAR
THROUGHOUT THE FORECAST PERIOD...BUT THE SHIPS FORECAST IS FOR ONLY
MODEST INTENSIFICATION.  IT SHOULD BE NOTED THAT THE GFS MODEL HAS
BEEN CONSISTENTLY UNDERESTIMATING THE SHEAR OVER ISIS FOR SEVERAL
DAYS NOW.  IF THE SHEAR DOES IN FACT SUBSIDE...GRADUAL
INTENSIFICATION SHOULD OCCUR AS THE THE SYSTEM BECOMES MORE COUPLED
IN THE VERTICAL.    
 
FORECASTER JARVINEN/HENNON
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/0300Z 17.4N 125.1W    45 KT
 12HR VT     13/1200Z 17.4N 126.5W    45 KT
 24HR VT     14/0000Z 17.4N 128.4W    50 KT
 36HR VT     14/1200Z 17.5N 129.9W    50 KT
 48HR VT     15/0000Z 17.6N 131.2W    55 KT
 72HR VT     16/0000Z 17.7N 132.7W    55 KT
 96HR VT     17/0000Z 17.9N 133.5W    55 KT
120HR VT     18/0000Z 18.1N 134.2W    55 KT
 
 
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