WDPN31 PGTW 190300 MSGID/GENADMIN/NAVPACMETOCCEN PEARL//JTWC// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (VIPA) WARNING NR 09// RMKS/ 1. FOR METEOROLOGISTS. 2. PROGNOSTIC REASONING FOR 190000Z0 TO 220000Z4 SEP 01. A. TYPHOON (TY) 21W (VIPA), LOCATED APPROXIMATELY 340 NM SOUTH- SOUTHWEST OF TOKYO, HAS TRACKED NORTHWARD AT 11 KNOTS DURING THE PAST SIX HOURS. THE WARNING POSITION IS BASED ON 182330Z7 INFRARED AND VISIBLE SATELLITE IMAGERY. THE WARNING INTENSITY IS BASED ON CURRENT INTENSITY ESTIMATES OF 75 KNOTS. ANIMATED SATELLITE IMAGERY DEPICTS A 9 NM ROUND EYE. B. TY 21W IS CURRENTLY UNDER THE STEERING INFLUENCE OF THE SUBTROPICAL RIDGE EAST OF THE SYSTEM. THE SYSTEM IS EXPECTED TO BEGIN EXTRA-TROPICAL (ET) TRANSITION AT THE 36 HOUR PERIOD AS IT BEGINS TO MERGE WITH AN APPROACHING BAROCLINIC ZONE. THE DYNAMIC AIDS CONSISTING OF NOGAPS (NGPS), GEOPHYSICAL FLUID DYNAMICS-NAVY (GFDN) MODEL, JAPANESE GLOBAL SPECTRAL (JGSM) AND TYPHOON (JTYM) MODELS, AND THE UK-MET OFFICE (EGRR) MODEL ARE IN FAIR AGREEMENT. GFDN AND EGRR ARE THE MAIN OUTLIERS TRACKING THE SYSTEM SLOWLY NORTHWARD AND THEN EASTWARD, WITH BOTH MODELS LOSING TRACK OF THE SYSTEM BY THE 48-HOUR FORECAST PERIOD. NGPS, JGSM, AND JTYM ARE IN GOOD AGREEMENT THROUGH THE ENTIRE FORECAST PERIOD ACCELERATING THE SYSTEM QUICKLY TO THE NORTHEAST AFTER THE 24-HOUR FORECAST PERIOD AS THE SYSTEM BEGINS ET. OUR FORECAST IS BASED ON THE NUMERICAL CONSENSUS CONSISTING OF THREE MODELS (NGAPS, JGSM, AND JTYM) TRACKING THE SYSTEM NORTHWARD FOR THE FIRST 24 HOURS, THEN NORTHEASTWARD FOR THE REMAINDER OF THE FORECAST PERIOD. C. TY 21W IS FORECAST TO INTENSIFY SLIGHTLY THROUGH 24 HOURS. THE SYSTEM IS THEN FORECAST TO WEAKEN AS IT ENCOUNTERS THE MID-LATITUDE WESTERLIES AND COOLER SEA SURFACE TEMPERATURES. D. WIND RADII ARE BASED ON CLIMATOLOGY FOR A SMALL TROPICAL CYCLONE. FORECAST TEAM: MAZANY/MORRIS/EDBERG//