697 
 WTPZ42 KNHC 082017
 TCDEP2
 TROPICAL STORM ISIS DISCUSSION NUMBER   3
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
 2 PM PDT WED SEP 08 2004
  
 VISIBLE AND MICROWAVE IMAGERY THIS AFTERNOON INDICATES THAT THE
 CYCLONE WAS A LITTLE BIT EAST OF EARLIER ESTIMATES...ON THE
 NORTHEAST EDGE OF THE DEEP CONVECTION.  THIS MEANS A SLOWER INITIAL
 MOTION ESTIMATE...275/6.  DVORAK INTENSITY ESTIMATES FROM BOTH TAFB
 AND SAB HAVE REACHED 35 KT...AND SO THE DEPRESSION IS UPGRADED TO
 TROPICAL STORM ISIS.
  
 THE 12Z MODELS APPEARED TO HAVE A BETTER INITIALIZATION OF THE
 VORTEX. THE GFDL...NOGAPS...AND GFS STILL START ISIS OFF TO THE
 NORTH OR NORTHWEST...AND THE LAST FEW IMAGES ARE SUGGESTIVE OF A
 NORTHWARD COMPONENT OF MOTION. IF THE STORM MAINTAINS VERTICAL
 CONTINUITY IT WOULD MOVE MORE WESTWARD WITH THE STRONG UPPER-LEVEL
 EASTERLY FLOW HOWEVER...AND THAT IS THE SCENARIO FOLLOWED BY THE
 OFFICIAL FORECAST.  REGARDLESS...ALL GUIDANCE MAINTAINS ENOUGH OF A
 SUBTROPICAL RIDGE NORTH OF ISIS TO PRODUCE A WEST-NORTHWEST TRACK
 AFTER 24 HOURS. THE OFFICIAL FORECAST IS IN BEST AGREEMENT WITH THE
 UKMET MODEL.  
  
 ISIS IS BATTLING 25 KT OF EASTERLY SHEAR...AND THE LOW-LEVEL CENTER
 IS SOMEWHAT ELONGATED NE-SW.  GLOBAL MODELS MAINTAIN OR
 EVEN INCREASE THE SHEAR SLIGHTLY AS THE UPPER-LEVEL HIGH OVER BAJA
 BUILDS SOUTHWESTWARD.  THE GFDL KEEPS ISIS AS A WEAK SYSTEM WHILE
 THE SHIPS GUIDANCE BRINGS THE CYCLONE UP TO ABOUT 60 KT.  IF ISIS
 SURVIVES THE NEXT 36 HOURS...IT COULD BECOME A STRONG TROPICAL
 STORM.  LATE IN THE FORECAST PERIOD ISISIS EXPECTED TO MOVE OVER
 SUB 26C WATER AND BEGIN TO SLOWLY WEAKEN.  AN ALTERNATIVE SCENARIO
 IS THAT ISIS BECOMES SHEARED AND DISSIPATES WITHIN THE NEXT DAY OR
 SO.
  
 FORECASTER FRANKLIN
   
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      08/2100Z 15.3N 111.3W    35 KT
  12HR VT     09/0600Z 15.7N 112.3W    35 KT
  24HR VT     09/1800Z 16.2N 114.0W    40 KT
  36HR VT     10/0600Z 16.9N 116.0W    45 KT
  48HR VT     10/1800Z 17.5N 118.0W    50 KT
  72HR VT     11/1800Z 18.5N 123.0W    55 KT
  96HR VT     12/1800Z 19.0N 129.0W    50 KT
 120HR VT     13/1800Z 19.5N 135.5W    40 KT
  
  
 $$