WDPN32 PGTW 091500 MSGID/GENADMIN/NAVPACMETOCCEN PEARL//JTWC// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (NARI) WARNING NR 15// RMKS/ 1. FOR METEOROLOGISTS. 2. PROGNOSTIC REASONING FOR 091200Z2 TO 121200Z6 SEP 01. A. TYPHOON (TY) 20W (NARI), LOCATED APPROXIMATELY 150 NM WEST- NORTHWEST OF KADENA, HAS REMAINED QUASI-STATIONARY DURING THE PAST SIX HOURS. THE WARNING POSITION IS BASED ON 091130Z4 INFRARED SATELLITE IMAGERY. THE WARNING INTENSITY IS BASED ON CURRENT SATELLITE INTENSITY ESTIMATES OF 77 KNOTS. ANIMATED INFRARED SATELLITE IMAGERY DEPICTS A BANDING 20 NM EYE WITH CYCLING CONVECTION. B. TY 20W IS CURRENTLY SITUATED WITHIN A WEAKNESS IN THE MID- LEVEL SUBTROPICAL RIDGE. TY 20W IS UNDER THE STEERING INFLUENCE OF THE WESTERN RIDGE WHICH EXTENDS NORTH-NORTHWEST OF THE SYSTEM TO APPROXIMATELY 124.0E7. THE DYNAMIC AIDS OF NOGAPS (NGPS), THE GEOPHYSICAL FLUID DYNAMICS-NAVY (GFDN) MODEL, THE UK-MET OFFICE (EGRR) MODEL, JMA TYPHOON (JTYM), AND GLOBAL SPECTRAL (JGSM) MODELS ARE IN GOOD AGREEMENT WITH A SLOW WEST-SOUTHWESTWARD TO SOUTHWESTWARD TRACK THROUGH THE PERIOD. THE ONLY EXCEPTION IS GFDN WHICH DRIFTS THE SYSTEM NORTHWARD. BASED ON THE LATEST UPPER-AIR ANALYSES AND SATELLITE IMAGERY, THERE IS GOOD CONFIDENCE IN THE FORECAST TRACK THROUGH THE NEXT 24-36 HOURS. HOWEVER, THE MID-LEVEL RIDGE IS EXPECTED TO WEAKEN WITH THE APPROACH OF A MID-LATITUDE TROUGH NEAR THE 24-36 HOUR POINT. AT THIS POINT THERE ARE TWO POSSIBLE SCENARIOS THAT MAY OCCUR. THE MORE LIKELY SCENARIO IS THAT THE SYSTEM WILL BECOME QUASI-STATIONARY AND THEN DRIFT NORTHEASTWARD. AN ALTERNATE SCENARIO IS THAT THE SYSTEM MAY TRACK WEST-SOUTHWESTWARD OVER TAIWAN, SIMILAR TO THE BETA ADVECTION MODEL SOLUTION. THE OFFICIAL FORECAST IS BASED ON THE NUMERICAL CONSENSUS OF THE FIVE AFOREMENTIONED MODELS. C. TY 20W IS EXPECTED TO MAINTAIN INTENSITY THROUGHOUT THE FORECAST PERIOD UNDER FAVORABLE CONDITIONS. D. WIND RADII FOR TY 20W ARE BASED ON CLIMATOLOGY FOR AN AVERAGE SIZE SYSTEM. FORECAST TEAM: MAZANY/PARKER/EDBERG//