WDPN31 PGTW 032100
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHOI-WAN) WARNING 
NR 021//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 04W (CHOI-WAN), LOCATED APPROXIMATELY 375 NM
SOUTH-SOUTHWEST OF TAIPEI, TAIWAN, HAS TRACKED NORTHWESTWARD AT 09 
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) 
SATELLITE IMAGERY SHOWS THE BROAD AREA OF CONVECTION ASSOCIATED WITH 
THE SYSTEM HAS BECOME TOTALLY OFFSET TO THE WEST-SOUTHWEST FROM A 
FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). THE INITIAL 
POSITION IS PLACED WITH HIGH CONFIDENCE USING A BULLSEYE 031352Z 
ASCAT-B PASS AND THE 031750Z PGTW SATELLITE FIX. THE INITIAL 
INTENSITY OF 35 KTS IS SET WITH GOOD CONFIDENCE BASED ON THE 
AFOREMENTIONED ASCAT-B PASS, AS WELL AS AN OVERALL ASSESSMENT OF 
DVORAK ESTIMATES OF T2.0/30KTS (PGTW), T2.9/43KTS (ADT), AND 
SATELLITE CONSENSUS (SATCON) OF 48KTS. THE SYSTEM IS IN A MARGINAL 
ENVIRONMENT WITH GOOD WESTWARD AND EQUATORWARD OUTFLOW AND VERY WARM 
(30-31C) SEA SURFACE TEMPERATURES, OFFSET BY MODERATE TO STRONG (20-
25KT) VERTICAL WIND SHEAR FROM THE NORTHEAST. THE CYCLONE IS 
TRACKING ALONG THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) 
TO THE EAST. 
3. FORECAST REASONING.
   A. THERE IS A CHANGE TO THE FORECAST PHILOSOPHY. THE FORECAST 
TRACK HAS BEEN REDUCED TO TAU 48 TO INDICATE DISSIPATION OVER WATER 
SOUTH OF KYUSHU, JAPAN.
   B. TS 04W HAS BEGUN TO MAKE ITS TURN NORTHEASTWARD AROUND THE 
AXIS OF THE STR AND AFTER TAU 12 WILL BEGIN TO WEAKEN DUE TO 
INTERACTION WITH THE SOUTHERN PORTION OF THE CHUNGYANG MOUNTAIN 
RANGE IN TAIWAN. BY TAU 24, TS 04W WILL WEAKEN TO 30 KTS DUE TO 
INCREASING VWS ASSOCIATED WITH STRONG NORTHEASTERLY FLOW FROM A 
SECONDARY STR ANCHORED OVER SOUTHEASTERN CHINA. BY TAU 36, THE 
SYSTEM WILL WEAKEN TO 25 KTS AS IT PASSES EAST-NORTHEAST OF KADENA 
AIR BASE. AT TAU 48, THE SYSTEM WILL BE FULLY DISSIPATED AS IT GETS 
ABSORBED INTO THE MEI-YU FRONTAL BOUNDARY APPROACHING FROM THE WEST, 
POSSIBLY SOONER. NUMERICAL MODELS ARE IN TIGHT AGREEMENT, SPREADING 
TO A MERE 58NM AT TAU 48, LENDING HIGH CONFIDENCE TO THE JTWC TRACK 
FORECAST.//
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