364 
 WTPZ42 KNHC 130237
 TCDEP2
 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
  
  
 $$