000
WTPZ45 KNHC 132051
TCDEP5
TROPICAL STORM ISELLE DISCUSSION NUMBER   2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP102008
200 PM PDT WED AUG 13 2008
 
VISIBLE AND INFRARED SATELLITE IMAGERY INDICATES THAT THE DEPRESSION
HAS ACQUIRED ENOUGH CONVECTIVE BANDING FEATURES... PRIMARILY IN THE
SOUTHERN SEMICIRCLE...FOR THE SYSTEM TO BE UPGRADED TO TROPICAL
STORM ISELLE.  THIS INTENSITY IS SUPPORTED BY A DVORAK INTENSITY
ESTIMATE OF T2.5/35 KT FROM TAFB.
 
THE INITIAL MOTION ESTIMATE IS 285/09 KT.  THERE IS NO SIGNIFICANT
CHANGE TO THE PREVIOUS FORECAST TRACK OR REASONING. TROPICAL STORM
ISELLE IS EXPECTED TO MOVE IN A GENERAL WEST-NORTHWESTWARD
DIRECTION THROUGH 72 HOURS...AND THEN TURN MORE WESTWARD AFTER THAT
AS THE CYCLONE WEAKENS AND IS STEERED MORE BY THE LOW-LEVEL
EASTERLIES. THE LATEST GFDL AND GFS MODEL RUNS TAKE A VERY WEAK
SYSTEM NORTHWARD TOWARD SOUTHERN BAJA CALIFORNIA AFTER 72
HOURS...WHEREAS THE REMAINDER OF THE NHC MODELS TAKE THE SYSTEM
SLOWLY WEST-NORTHWESTWARD THEN WESTWARD. THE OFFICIAL FORECAST
TRACK REMAINS ALONG THE EXTREME SOUTHERN PORTION OF THE GUIDANCE
ENVELOPE.  
 
AN APPROACHING UPPER-LEVEL JET MAXIMUM TO THE NORTHEAST OF ISELLE IS
ALREADY BEGINNING TO INCREASE THE VERTICAL WIND SHEAR ACROSS THE
CYCLONE. THE SHIPS INTENSITY MODEL MAINTAINS 20-25 KT OF SHEAR
ACROSS ISELLE THROUGHOUT THE PERIOD...SO ONLY MODEST STRENGTHENING
IS FORECAST. BY 96 HOURS...ISELLE WILL BE MOVING OVER COOLER WATERS
THAT ARE EXPECTED TO INDUCE GRADUAL WEAKENING. THE OFFICIAL
INTENSITY FORECAST IS SLIGHTLY LOWER THAN THE SHIPS MODEL.

 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/2100Z 16.6N 106.7W    35 KT
 12HR VT     14/0600Z 17.1N 108.1W    40 KT
 24HR VT     14/1800Z 17.5N 109.7W    45 KT
 36HR VT     15/0600Z 17.9N 111.2W    45 KT
 48HR VT     15/1800Z 18.2N 112.5W    45 KT
 72HR VT     16/1800Z 18.6N 114.5W    45 KT
 96HR VT     17/1800Z 19.0N 116.5W    40 KT
120HR VT     18/1800Z 19.0N 118.5W    35 KT
 
$$
FORECASTER STEWART/FRANKLIN