WDPN31 PGTW 310900
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHOI-WAN) WARNING 
NR 007//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
      TROPICAL STORM 04W (CHOI-WAN), LOCATED APPROXIMATELY 267 NM 
NORTHWEST OF SONSOROL, HAS TRACKED NORTHWESTWARD AT 11 KNOTS OVER 
THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) 
SHOWS THE LOW-LEVEL CIRCULATION (LLC) IS PARTIALLY EXPOSED, 
OFFSETTING THE REGION OF DEEPENED CENTRAL CONVECTION WITH 
OVERSHOOTING CLOUD TOPS JUST SOUTHWEST OF THE MAIN LLC. THE INITIAL 
POSITION IS PLACED WITH HIGH CONFIDENCE USING EXTRAPOLATION WITH A 
310455Z AMSR2 89GHZ IMAGE AND THE EXPOSED LLC FROM THE MSI LOOP. THE 
INITIAL INTENSITY OF 50KTS IS HEDGED HIGHER THAN MULTIPLE AGENCY, 
AND OBJECTIVE DVORAK ESTIMATES RANGING FROM 30KTS TO 45KTS. 
ENVIRONMENTAL ANALYSIS INDICATES THE SYSTEM RESIDES IN A SEMI-
HOSTILE ENVIRONMENT.   FAIR EASTERLY OUTFLOW ALONG WITH WARM (30-
31C) SEA SURFACE TEMPERATURES (SSTS) ARE BEING OFFSET BY MODERATE 
(20-25KT) NORTHEASTERLY VERTICAL WIND SHEAR (VWS) AND DRY AIR-
ENTRAINING, AS SHOWN ON THE TOTAL PRECIPITABLE WATER LOOP.  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 OVER THE WARM PHILIPPINE SEA UP TO TAU 72.  
THE SYSTEM IS FORECAST TO STEADILY INTENSIFY AS CONDITIONS WILL 
BEGIN TO SLIGHTLY IMPROVE ONCE IT APPROACHES THE RIDGE AXIS, PEAKING 
TO 70KTS BY TAU 48. NUMERICAL MODELS ARE IN TIGHT AGREEMENT, AND 
GRADUALLY SPREAD FROM 72NM AT TAU 12 TO 270NM BY TAU 72. NUMERICAL 
MODEL GUIDANCE HAS IMPROVED OVER THE LAST SIX HOURS, LENDING FAIR 
CONFIDENCE TO THE JTWC TRACK FORECAST THAT IS LAID SLIGHTLY RIGHT OF 
THE MODEL CONSENSUS, OFFSETTING THE FAR LEFT OUTLIER OF GFS.
   C. AFTER TAU 72, TS 04W WILL CREST THE STR AXIS AND TURN 
NORTHWARD, THEN ACCELERATE NORTHEASTWARD. AS TS 04W TRANSITS TO THE 
NORTHEAST, IT WILL MOVE IN A REGION OF HIGHER VWS AS IT APPROACHES 
THE MEI-YU BOUNDARY IN THE PREVAILING WESTERLIES. THE SYSTEM WILL 
BEGIN ERODING DUE TO COOLING SSTS AND DECREASE INTENSITY TO 55KTS BY 
TAU 96. CONCURRENTLY AT TAU 96, THE CYCLONE WILL BEGIN EXTRA-
TROPICAL TRANSITION AS IT ENTERS THE COLD BAROCLINIC ZONE AND 
COMPLETE TRANSITION BY TAU 120. NUMERICAL MODELS REMAIN IN GOOD 
AGREEMENT, LENDING ALSO FAIR CONFIDENCE IN THE EXTENDED PORTION OF 
THE JTWC FORECAST TRACK.//
NNNN