WDPN31 PGTW 010300
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHOI-WAN) WARNING 
NR 010//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 04W (CHOI-WAN), LOCATED APPROXIMATELY 457 NM 
SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WEST-NORTHWESTWARD AT 
09 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE 
IMAGERY DEPICTS A FULLY-EXPOSED LOW-LEVEL CIRCULATION WITH 
PERSISTENT DEEP CONVECTION SHEARED OVER THE SOUTHWEST QUADRANT DUE 
TO MODERATE TO STRONG (20-25 KNOTS) NORTHEASTERLY VERTICAL WIND 
SHEAR (VWS) ASSOCIATED WITH A TUTT CELL NEAR 13N 130E. A 312228Z 
SSMIS 91GHZ MICROWAVE IMAGE INDICATES AN INCREASINGLY DISORGANIZED 
SYSTEM WITH FRAGMENTED BANDING OVER THE WESTERN SEMICIRCLE, WHICH IS 
LOCATED OVER LAND. THE INITIAL INTENSITY IS ASSESSED CONSERVATIVELY 
AT 40 KNOTS HEDGED ABOVE PGTW/RJTD DVORAK CURRENT INTENSITY 
ESTIMATES OF 2.5 (35 KNOTS) BASED ON RECENT ASCAT DATA. THE SYSTEM 
IS LOCATED WITHIN A MARGINALLY-FAVORABLE ENVIRONMENT WITH 
NORTHEASTERLY VERTICAL WIND SHEAR (VWS) OFFSET BY GOOD EQUATORWARD 
OUTFLOW AND WARM SST VALUES (29-30C). TS 04W HAS DECOUPLED AND IS 
TRACKING WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE OF A DEEP-
LAYERED SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTH AND 
NORTHEAST.
3. FORECAST REASONING.
   A. DUE TO THE WESTWARD SHIFT IN THE FORECAST TRACK OVER THE 
PHILIPPINES THROUGH TAU 72, THE INTENSITY FORECAST IS SIGNIFICANTLY 
LOWER THAN THE PREVIOUS FORECAST.  
   B. TS 04W IS FORECAST TO TRACK ALONG THE EASTERN COAST OF THE 
PHILIPPINES WHILE MAINTAINING INTENSITY AT 40 KNOTS THROUGH TAU 36. 
AFTER TAU 36, FURTHER WEAKENING WILL OCCUR AS THE SYSTEM TRACKS OVER 
THE MOUNTAINOUS TERRAIN OF LUZON. WITH THE EXCEPTION OF GFS AND 
NAVGEM, WHICH TRACK THE SYSTEM INTO THE SOUTH CHINA SEA, THE BULK OF 
THE GUIDANCE SUPPORTS THE JTWC FORECAST WITH MODERATE CONFIDENCE. 
ADDITIONALLY, THE 31/18Z ECMWF ENSEMBLE INDICATES INCREASED 
POTENTIAL FOR A TRACK OVER LUZON WITH THE BULK OF THE ENSEMBLE 
MEMBERS DIRECTLY OVER LUZON.
   C. IN THE EXTENDED PERIOD, THE WESTERN PORTIONS OF THE STR WILL 
CONTINUE TO ERODE ALLOWING THE SYSTEM TO RECURVE NORTHEASTWARD. TS 
04W WILL WEAKEN AS IT ENCOUNTERS INCREASING VWS ASSOCIATED WITH A 
MIDLATITUDE SHORTWAVE TROUGH EXPECTED TO DIG OVER THE EAST CHINA 
SEA. TS 04W IS EXPECTED TO BEGIN EXTRA-TROPICAL TRANSITION (ETT) 
NEAR TAU 96 BUT MAY DISSIPATE PRIOR TO COMPLETING ETT NEAR TAU 120. 
MODEL GUIDANCE DIVERGES WITH A LARGE SPREAD IN SOLUTIONS AT TAU 120. 
THE JTWC FORECAST REMAINS POSITIONED CLOSE TO THE MULTI-MODEL 
CONSENSUS. BASED ON THE UNCERTAINTY IN THE TIMING AND DEGREE OF 
RECURVATURE, THERE IS MODERATE CONFIDENCE IN THE JTWC FORECAST 
TRACK.//
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