WDPN31 PGTW 181500 
MSGID/GENADMIN/NAVPACMETOCCEN PEARL HARBOR HI/JTWC// 
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W WARNING NR/ 
13// 
RMKS/ 
1. FOR METEOROLOGISTS. 
2. PROGNOSTIC REASONING FOR 181200Z2 AUG TO 211200Z6 AUG 02. 
   A. TROPICAL STORM (TS) 20W (VONGFONG), LOCATED APPROXIMATELY 365 
NM SOUTH-SOUTHWEST OF HONG KONG, CHINA, HAS TRACKED WEST-
NORTHWESTWARD AT 06 KNOTS OVER THE PAST 6 HOURS. THE WARNING 
POSITION IS BASED ON 181130Z4 ENHANCED INFRARED SATELLITE IMAGERY. 
THE WARNING INTENSITY IS BASED ON SATELLITE CURRENT INTENSITY 
ESTIMATES OF 30 AND 35 KNOTS AND SYNOPTIC OBSERVATIONS. ANIMATED 
ENHANCED INFRARED IMAGERY INDICATES INCREASING DEEP CONVECTION OVER 
THE LOW LEVEL CIRCULATION CENTER AND WEAK RAIN BANDS FORMING IN THE 
NORTHERN SEMI-CIRCLE.
   B. SOUTHWESTERLIES ASSOCIATED WITH THE MONSOON SURGE ARE EXPECTED 
TO CONTINUE TO ENHANCE CONVECTION ALONG THE SOUTHWESTERN QUADRANT OF 
TS 20W OVER THE NEXT 12 HOURS. INTERACTION WITH THE SOUTHWESTERLIES 
WILL CAUSE THE SYSTEM TO SLOW DOWN INITIALLY. THE RECENT WEST-
NORTHWESTWARD MOTION SUGGESTS THAT THE SYSTEM IS BEING STEERED BY 
WEAK LOW LEVEL RIDGING EXTENDING ACROSS THE TAIWAN TRAIT. A WEAK 
MID-LEVEL RIDGE IS EXPECTED TO CONTINUE TO BUILD EAST OF THE SYSTEM, 
EXTENDING OVER TAIWAN AND INTO THE SOUTH CHINA SEA. AS THIS RIDGE 
BUILDS, TS 20W WILL TRACK NORTHWESTWARD THROUGH 72 HOURS. THE 
AVAILABLE DYNAMIC AIDS, CONSISTING OF THE NOGAPS (NGPS), UKMET 
(EGRR), JAPANESE GLOBAL (JGSM) AND TYPHOON (JTYM) MODELS, AVIATION 
(AVN), GFDN, COAMPS, AND AFWA MM5, ARE IN FAIR AGREEMENT WITH TRACK 
BUT VARY IN SPEED. AS THE SYSTEM TRACKS OVER LAND, COAMPS, JTYM, 
GFDN, EGRR, AND JGSM DO NOT RESOLVE THE LOW LEVEL WELL, JUMPING TO 
ANOTHER WEAK LOW LEVEL CIRCULATION. OUR FORECAST IS BASED ON 
PERSISTENCE AND A BLEND OF ALL AVAILABLE DYNAMIC AIDS WITH LESS 
EMPHASIS ON THE AVN AND JTYM. 
   C. TS 20W IS FORECAST TO DEVELOP AT A LESS THAN CLIMATOLOGICAL 
RATE THROUGH 36 HOURS WHILE UNDER FAVORABLE UPPER LEVEL CONDITIONS. 
BY 72 HOURS, TS 20W IS EXPECTED TO WEAKEN AS THE SYSTEM TRACKS OVER 
LAND.
   D. FORECAST WIND RADII ARE BASED ON CLIMATOLOGY FOR AN AVERAGED 
SIZED SYSTEM. 
FORECAST TEAM: WINGEART/LAM/KUMAGA/STATEN//
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