WDPN32 PGTW 130300 
MSGID/GENADMIN/NAVPACMETOCCEN PEARL//JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (NARI) WARNING 
NR 29//
RMKS/
1. FOR METEOROLOGISTS.
2. PROGNOSTIC REASONING FOR 130000Z4 TO 160000Z7 SEP 01.
   A. TYPHOON (TY) 20W (NARI), LOCATED APPROXIMATELY 70 NM WEST OF 
KADENA, HAS TRACKED WEST-NORTHWESTWARD AT 4 KNOTS DURING THE PAST 
SIX HOURS. THE WARNING POSITION IS BASED ON 122330Z1 INFRARED AND 
VISIBLE SATELLITE IMAGERY, A 122155Z6 SPECIAL SENSOR MICROWAVE IMAGE 
(SSMI), RADAR REPORTS, AND AVAILABLE SYNOPTIC DATA. THE WARNING 
INTENSITY IS BASED ON SATELLITE CURRENT INTENSITY ESTIMATES OF 65 
AND 81 KNOTS, AND SYNOPTIC REPORTS. ANIMATED WATER VAPOR IMAGERY 
REVEALS A MID-LEVEL RIDGE BUILDING OVER CENTRAL CHINA WHICH WILL BE 
THE MECHANISM TO STEER TY 20W WEST-NORTHWESTWARD THEN SOUTHWESTWARD 
THROUGH THE FORECAST PERIOD.
   B. TY 20W IS FORECAST TO REMAIN WITHIN A WEAKNESS IN THE 
SUBTROPICAL RIDGE EARLY IN THE FORECAST PERIOD. A MID-LEVEL RIDGE 
BUILDING OVER CENTRAL CHINA WILL PROVIDE THE STEERING FLOW TO PUSH 
THE SYSTEM WEST-NORTHWESTWARD. AS THE RIDGE MOVES EAST AND THE 
SYSTEM MOVES SOUTH OF THE RIDGE AXIS, A WEST-SOUTHWESTWARD MOTION IS 
EXPECTED MIDWAY THROUGH THE FORECAST PERIOD. FORECAST SOLUTIONS ARE 
SHOWING IMPROVED AGREEMENT, BUT HAVE TWO OUTLIERS WORTH NOTING. THE 
UK-MET GLOBAL (EGRR) MODEL TRACKS THE SYSTEM NORTH-NORTHWESTWARD,
WHICH JTWC ATTRIBUTES TO EXCESSIVE DEVELOPMENT OF THE RIDGE EAST OF 
TY 20W. THE JMA GLOBAL SPECTRAL (JGSM) MODEL AGREES WITH THE 
CONSENSUS TRACK, BUT ACCELERATES THE SYSTEM AFTER 48 HOURS. JTWC 
ATTRIBUTES THIS TO EXCESSIVE DEVELOPMENT OF THE MID-LEVEL RIDGE OVER
CHINA. THE JMA TYPHOON (JTYM) MODEL, GEOPHYSICAL FLUID DYNAMIC-NAVY
(GFDN) MODEL, AND NOGAPS (NGPS) ARE IN GOOD AGREEMENT. THE NUMERICAL
CONSENSUS CONSISTS OF FIVE MODELS (NOGAPS, GFDN, JGSM, JTYM, AND 
EGRR). OUR FORECAST IS BASED ON THE NUMERICAL CONSENSUS AND AN 
ANALYSIS OF THE CURRENT STEERING ENVIRONMENT. 
   C. TY 20W IS EXPECTED TO SLOWLY WEAKEN AS THE SYSTEM TRACKS
OVER COOLER SEA SURFACE TEMPERATURES (SST) CREATED BY UPWELLING
ASSOCIATED WITH THE SYSTEM AND INTO A LESS FAVORABLE SHEAR 
ENVIRONMENT LATE IN THE FORECAST PERIOD.
   D. WIND RADII FOR TY 20W ARE BASED ON SYNOPTIC DATA, A RECENT 
QUIKSCAT GALE WIND ANALYSIS, AND CLIMATOLOGY FOR A SMALL SIZED 
CYCLONE. 
FORECAST TEAM: JEFFRIES/VILPORS/WAUGAMAN/SHERRY