000 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