WDPN31 PGTW 181500 
MSGID/GENADMIN/NAVPACMETOCCEN PEARL//JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (VIPA)WARNING NR 
07//
RMKS/
1. FOR METEOROLOGISTS.
2. PROGNOSTIC REASONING FOR 181200Z2 TO 211200Z6 SEP 01.
   A. TROPICAL STORM (TS) 21W (VIPA), LOCATED APPROXIMATELY 495 NM 
SOUTH OF TOKYO, HAS TRACKED NORTHWARD AT 11 KNOTS DURING THE PAST 
SIX HOURS. THE WARNING POSITION IS BASED ON 181130Z4 INFRARED 
SATELLITE IMAGERY. THE WARNING INTENSITY IS BASED ON CURRENT 
INTENSITY ESTIMATES OF 45 KNOTS AND A 181118Z0 SPECIAL SENSOR 
MICROWAVE IMAGER PASS (SSMI). ANIMATED SATELLITE IMAGERY DEPICTS 
IMPROVING ORGANIZATION OVER THE PAST SIX HOURS WITH THE SSMI PASS 
INDICATING THE DEVELOPMENT OF A POSSIBLE EMBEDDED EYE. 
   B. TS 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 48 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 IS THE MAIN OUTLIER TRACKING THE SYSTEM SLOWLY NORTH THROUGH 48 
HOURS AND THEN EASTWARD THROUGH THE 72 HOUR PERIOD. EGRR, JGSM AND 
JTYM ARE IN GOOD AGREEMENT THROUGH 36 HOURS AND THEN ACCELERATE THE 
SYSTEM QUICKLY TO THE NORTHEAST AFTER THIS POINT. OUR FORECAST IS 
BASED ON THE NUMERICAL CONSENSUS CONSISTING OF FIVE MODELS (NGAPS, 
GFDN, JGSM, JTYM, AND EGRR) TRACKING THE SYSTEM NORTHWARD FOR THE 
FIRST 24 HOURS, THEN NORTHEASTWARD FOR THE REMAINDER OF THE FORECAST 
PERIOD.
   C. TS 21W IS FORECAST TO INTENSIFY 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 AN AVERAGE SIZE 
TROPICAL CYCLONE AND A RECENT QUIKSCAT GALE WIND ANALYSIS.  
FORECAST TEAM: CANTRELL/VILPORS/WAUGAMAN/SHERRY//