WDPN31 PGTW 311500
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHOI-WAN) WARNING 
NR 008//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 04W (CHOI-WAN), LOCATED APPROXIMATELY 307 NM 
NORTHWEST OF SONSOROL, PALAU, HAS TRACKED NORTHWESTWARD AT 07 KNOTS 
OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE 
IMAGERY (EIR) SHOWS THE MAIN CONVECTION CONTINUES TO REMAIN DETACHED 
FROM THE PARTIALLY EXPOSED LOW LEVEL CIRCULATION. THE INITIAL 
POSITION IS PLACED WITH HIGH CONFIDENCE USING A WELL TIMED BULLSEYE 
311156Z METOP-A ASCAT IMAGE AND THE EIR LOOP. THE INITIAL INTENSITY 
OF 50KTS IS HEDGED HIGHER THAN MULTIPLE AGENCY, AND OBJECTIVE DVORAK 
ESTIMATES RANGING FROM 30KTS TO 45KTS, AND THE ASCAT IMAGE. 
ENVIRONMENTAL ANALYSIS INDICATES THE SYSTEM RESIDES IN A SEMI-
HOSTILE ENVIRONMENT. FAIR EASTERLY OUTFLOW AND WARM (30-31C) SEA 
SURFACE TEMPERATURES (SSTS) ARE CURRENTLY BEING OFFSET BY MODERATE 
TO STRONG (25-30KT) NORTHEASTERLY VERTICAL WIND SHEAR (VWS) AND DRY 
AIR-ENTRAINING ALONG THE NORTHERN SIDE OF THE SYSTEM. THE CYCLONE IS 
TRACKING ALONG THE SOUTHWEST PERIPHERY OF A DEEP-LAYERED SUBTROPICAL 
RIDGE (STR) TO THE NORTHEAST. 
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. UNDER THE STEERING INFLUENCE OF THE STR, TS 04W WILL CONTINUE 
TO TRACK NORTHWESTWARD, REACHING THE EASTERN SIDE OF THE PHILIPPINE 
ISLANDS IN THE WARM PHILIPPINE SEA. THE SYSTEM IS FORECAST TO SLOWLY 
INTENSIFY AS CONDITIONS WILL BEGIN SLIGHTLY IMPROVING ONCE IT 
APPROACHES THE RIDGE AXIS NEARING THE EAST COAST OF LUZON WITH A 
PEAK OF 65KTS BY TAU 48. NUMERICAL MODELS ARE IN TIGHT AGREEMENT AND 
SPREAD FROM 65NM AT TAU 12 TO 330NM BY TAU 72, LENDING FAIR 
CONFIDENCE TO THE JTWC TRACK FORECAST THAT IS LAID SLIGHTLY RIGHT OF 
THE MODEL CONSENSUS, OFFSETTING THE FAR LEFT OUTLIER OF NAVGEM.
   C. AFTER TAU 72, TS 04W WILL CREST THE STR AXIS, TURN AND 
ACCELERATE NORTHEASTWARD. AS TS 04W TRANSITS TO THE NORTHEAST, IT 
WILL MOVE INTO A REGION OF HIGHER VWS AS IT APPROACHES THE MEI-YU 
BOUNDARY IN THE PREVAILING WESTERLIES. THE SYSTEM WILL THEN BEGIN 
ERODING MORE RAPIDLY DUE TO COOLING SSTS AND DECREASE INTENSITY TO 
50KTS BY TAU 96. CONCURRENTLY AT TAU 96, THE CYCLONE WILL BEGIN 
EXTRA-TROPICAL TRANSITION AS IT ENTERS THE COLD BAROCLINIC ZONE AND 
COMPLETE TRANSITION THRU TAU 120. NUMERICAL MODELS REMAIN IN FAIR 
AGREEMENT, LENDING FAIR CONFIDENCE TO THE EXTENDED PORTION OF THE 
JTWC FORECAST TRACK.//
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