929 
 WTPZ42 KNHC 220242
 TCDEP2
 TROPICAL STORM IVO DISCUSSION NUMBER  15
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP122007
 800 PM PDT FRI SEP 21 2007
  
 THE CONVECTIVE PATTERN ASSOCIATED WITH IVO HAS RAPIDLY DETERIORATED
 OVER THE PAST SEVERAL HOURS WITH THE FORMATION OF A SHARP WESTERN
 EDGE AND WARMING CLOUD TOP TEMPERATURES...LIKELY DUE TO INCREASING
 SOUTHWESTERLY SHEAR. A FORTUITOUS 0028 UTC SSM/I PASS INDICATED
 THAT THE LOW-LEVEL CENTER IS LOCATED WELL TO THE SOUTHWEST OF THE
 DEEPEST CONVECTION AND THE INITIAL INTENSITY IS SET AT 55 KT IN
 ACCORDANCE WITH A DROP IN T-NUMBERS...BUT THIS ESTIMATE IS PROBABLY
 GENEROUS.
 
 IVO IS STILL MOVING RATHER SLOWLY BUT THE SSM/I PASS SUGGESTS THAT
 THE SYSTEM HAS TAKEN A SLIGHT TURN TO THE RIGHT OF THE EXTRAPOLATED
 TRACK...WITH AN INITIAL MOTION OF 030/5. MODEL GUIDANCE REMAINS
 SPLIT IN TWO CAMPS REGARDING THE FORECAST TRACK OF IVO. THE UKMET
 AND NOGAPS MODELS REMAIN THE NORTHERN OUTLIERS BY MOVING A DEEP
 TROPICAL SYSTEM ACROSS THE SOUTHERN PART OF BAJA CALIFORNIA IN
 ABOUT 36-48 HOURS. HOWEVER...CONSIDERING THAT VERTICAL SHEAR HAS
 ALREADY BECOME A MAJOR FACTOR IN THE WEAKENING OF IVO...THE
 GUIDANCE PROVIDED BY THE GFS...ECMWF...AND GFDL MODELS APPEAR MORE
 REASONABLE AT THIS TIME. THESE MODELS MOVE THE SHEARED MID-LEVEL
 CIRCULATION OF IVO TO THE NORTHEAST ACROSS BAJA WITH THE LOW-LEVEL
 CIRCULATION TURNING EAST AND STAYING SOUTH OF BAJA...POSSIBLY
 DISSIPATING IN 3-4 DAYS. THEREFORE...THE OFFICIAL FORECAST IS
 ADJUSTED TOWARD THE SOUTHERN CLUSTER OF MODELS AND SHOWS VERY SLOW
 MOTION AT DAYS 4 AND 5.
 
 THE INTENSITY FORECAST SHOWS A GRADUAL WEAKENING THROUGH THE
 FORECAST PERIOD BUT IS NOT AS FAST AS SHIPS GUIDANCE...JUST IN CASE
 THE SYSTEM CONTINUES TO GENERATE CONVECTION AS IT MOVES TOWARDS
 BAJA. IF THE SHEAR REALLY IS MAKING A KNOCK-OUT PUNCH...WHICH
 APPEARS POSSIBLE GIVEN THE SATELLITE REPRESENTATION...THEN IVO
 WOULD WEAKEN FASTER THAN INDICATED IN THE OFFICIAL FORECAST.
 
 ALTHOUGH IVO IS EXPECTED TO WEAKEN AS IT APPROACHES SOUTHERN BAJA
 CALIFORNIA...TROPICAL STORM WATCHES MAY BE REQUIRED TOMORROW.
  
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      22/0300Z 20.4N 113.1W    55 KT
  12HR VT     22/1200Z 21.2N 112.8W    50 KT
  24HR VT     23/0000Z 22.0N 112.3W    45 KT
  36HR VT     23/1200Z 22.6N 111.8W    40 KT
  48HR VT     24/0000Z 23.1N 111.3W    35 KT
  72HR VT     25/0000Z 24.2N 110.3W    25 KT...INLAND
  96HR VT     26/0000Z 24.5N 110.0W    25 KT
 120HR VT     27/0000Z 25.0N 109.5W    20 KT...REMNANT LOW
  
 $$
 FORECASTER BERG/BLAKE