Tropical Storm ISELLE

ZCZC MIATCDEP5 ALL
TTAA00 KNHC DDHHMM
TROPICAL STORM ISELLE DISCUSSION NUMBER   3
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP102008
800 PM PDT WED AUG 13 2008
 
AN ASCAT PASS AT 1630Z THIS AFTERNOON PLACED THE CENTER OF ISELLE ON
THE NORTHEASTERN EDGE OF A SMALL BLOB OF CONVECTION...FARTHER NORTH
THAN PREVIOUSLY ESTIMATED...AND LACKING ANY EVIDENCE TO THE
CONTRARY I'M ASSUMING THAT RELATIVE LOCATION STILL HOLDS.  THIS
REQUIRES A NORTHWARD ADJUSTMENT OF THE PREVIOUS TRACK WITH AN
INITIAL MOTION ESTIMATE OF 295/10.  ISELLE IS CURRENTLY LOCATED TO
THE SOUTH OF MID- TO UPPER-LEVEL HIGH PRESSURE OVER NORTH-CENTRAL
MEXICO...BUT THE TRACK FORECAST IS UNUSUALLY DIFFICULT...AS ISELLE
IS IN A HIGH-SHEAR ENVIRONMENT AND THE GUIDANCE IS HIGHLY DIVERGENT. 
MOST OF THE GUIDANCE CALLS FOR A CONTINUED TRACK TO THE
WEST-NORTHWEST OR NORTHWEST FOR AT LEAST THE NEXT COUPLE OF DAYS AS
ISELLE APPROACHES THE WESTERN PERIPHERY OF THE AFOREMENTIONED HIGH
PRESSURE.  THE GFDL...HOWEVER...CALLS FOR ISELLE TO SUCCUMB TO THE
SHEAR WITHIN A DAY OR SO...TAKING ITS MID-LEVEL VORTICITY WESTWARD
AND ITS LOW-LEVEL REMNANTS NORTHWARD TOWARD THE BAJA PENINSULA. 
THE LATTER TRACK SCENARIO IS BEING DISCOUNTED FOR NOW...NOT SO MUCH
BECAUSE IT IS UNLIKELY...BUT BECAUSE IF SUCH A TRACK DOES
MATERIALIZE THERE SHOULD BE VERY LITTLE OF ISELLE LEFT.  IF ISELLE
IS STILL A TROPICAL CYCLONE IN 48-72 HOURS...IT WILL HAVE HAD TO
FOLLOW A DEEPER LAYER FLOW AND THAT IS THE ASSUMPTION REFLECTED IN
THE TRACK GIVEN BELOW.  THE OFFICIAL FORECAST HAS BEEN SHIFTED
NORTHWARD OF THE PREVIOUS ADVISORY TRACK AND IS IN BEST AGREEMENT
WITH THE GFS AND MEDIUM BAM THROUGH 48 HOURS...AND SHOWS RELATIVELY
LITTLE MOTION AFTER THAT IN DEFERENCE TO THE MODEL SPREAD.   IT
SHOULD BE NOTED THAT THE MODEL CONSENSUS LIES EVEN FARTHER TO THE
NORTH AND EAST OF THE OFFICIAL FORECAST.

THE INITIAL INTENSITY REMAINS 35 KT...WITH DVORAK INTENSITY
ESTIMATES OF 35 AND 45 KT FROM SAB AND TAFB...RESPECTIVELY. 
NORTHEASTERLY OR EASTERLY SHEAR IS FORECAST BY THE GLOBAL MODELS TO
CONTINUE FOR MOST OF THE FIVE-DAY FORECAST PERIOD...AND NONE OF THE
AVAILABLE INTENSITY GUIDANCE SUGGESTS MUCH STRENGTHENING.  THE
OFFICIAL FORECAST CALLS FOR VERY LITTLE CHANGE IN STRENGTH UNTIL
AFTER 72 HOURS WHEN ISELLE IS EXPECTED TO CROSS THE 26C SST
ISOTHERM.  AS NOTED ABOVE...IT IS QUITE POSSIBLE THAT ISELLE WILL
NOT LAST THAT LONG.   
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      14/0300Z 17.4N 107.7W    35 KT
 12HR VT     14/1200Z 17.9N 108.9W    40 KT
 24HR VT     15/0000Z 18.4N 110.3W    40 KT
 36HR VT     15/1200Z 19.0N 111.3W    40 KT
 48HR VT     16/0000Z 19.8N 112.5W    40 KT
 72HR VT     17/0000Z 20.5N 114.0W    40 KT
 96HR VT     18/0000Z 21.0N 115.5W    35 KT
120HR VT     19/0000Z 21.0N 117.0W    30 KT
 
$$
FORECASTER FRANKLIN
 
NNNN