WDPN31 PGTW 041500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 01W (SONAMU) WARNING NR 
05//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 01W (SONAMU), LOCATED APPROXIMATELY 430 NM 
EAST-SOUTHEAST OF HO CHI MINH CITY, VIETNAM, HAS TRACKED WESTWARD AT 
14 KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED (IR) SATELLITE 
IMAGERY SHOWS CENTRAL CONVECTION PERSISTING OVER THE LOW LEVEL 
CIRCULATION CENTER AND SOME FRAGMENTED FORMATIVE BANDING OVER THE 
NORTHERN SEMI-CIRCLE. THE INITIAL POSITION IS BASED UPON THE IR LOOP 
AND IS EXTRAPOLATED FROM A 041011Z SSMIS 37 GHZ MICROWAVE IMAGE WITH 
FAIR CONFIDENCE. THE INITIAL INTENSITY IS BASED UPON CONGRUENT 
DVORAK INTENSITY ESTIMATES OF 35 KNOTS FROM PGTW, KNES AND RJTD. 
UPPER-LEVEL ANALYSIS REVEALS TS 01W IS WITHIN A MARGINAL UPPER-LEVEL 
ENVIRONMENT WITH MODERATE TO STRONG (20 TO 30 KNOTS) EASTERLY 
VERTICAL WIND SHEAR (VWS) OFFSET BY STRONG DIFFLUENCE AND GOOD 
POLEWARD OUTFLOW INTO THE UPPER-LEVEL WESTERLIES. TS 01W IS 
CURRENTLY TRACKING WESTWARD UNDER THE DEEP-LAYERED SUBTROPICAL RIDGE 
(STR) THAT IS ENTRENCHED OVER THE WESTERN PACIFIC AND SOUTH CHINA 
SEA.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. TS 01W WILL CONTINUE TO TRACK WESTWARD THROUGH TAU 48 UNDER 
THE STEERING INFLUENCE OF THE STR POSITIONED OVER THE NORTHERN SOUTH 
CHINA SEA. TS 01W IS FORECAST TO INTENSIFY TO A PEAK OF 45 KNOTS BY 
TAU 24 AND THEN WEAKEN AS HIGHER VWS IS ENCOUNTERED DUE TO A 
FORECAST NORTHEAST SURGE. DYNAMIC MODEL GUIDANCE IS IN GOOD 
AGREEMENT WITH SOLUTIONS VARYING FROM A WESTWARD TRACK TO A WEST-
SOUTHWESTWARD TRACK WITH THE DIFFERENCES DUE TO THE INTERACTION WITH 
THE NORTHEAST SURGE FLOW AND THE INTENSITIES AS THE SYSTEM CROSSES 
THE SOUTH CHINA SEA. THE JTWC FORECAST IS POSITIONED CLOSE TO THE 
MODEL CONSENSUS, FAVORING THE ECMWF AND GFS SOLUTIONS. DUE TO THE 
GOOD AGREEMENT AMONG THE MODEL SOLUTIONS, THERE IS HIGH CONFIDENCE 
IN THIS PORTION OF THE FORECAST.
   C. IN THE EXTENDED TAUS, TS 01W WILL TURN SOUTHWESTWARD UNDER THE 
STEERING INFLUENCE OF A NEAR-EQUATORIAL RIDGE (NER), FORECAST TO 
BUILD IN OVER INDONESIA. TS 01W WILL CONTINUE TO WEAKEN UNDER THE 
UNFAVORABLE UPPER-LEVEL ENVIRONMENT AND DISSIPATE BY TAU 96 AS THE 
SYSTEM APPROACHES THE COAST OF MALAYSIA. DYNAMIC MODEL GUIDANCE 
DIVERGES IN THE EXTENDED TAUS WITH NOGAPS, AND GFDN TAKING A FASTER 
AND STRAIGHT TRACK, WHILE GFS, ECMWF, AND JGSM TAKE A SLOWER TRACK 
WITH A SOUTHWESTERN TURN. THE JTWC FAVORS THE LATTER SOLUTIONS BASED 
UPON THE EXPECTED WEAKENING AND THE STEERING INFLUENCE FROM THE NER 
AND THE NORTHEAST FLOW. DUE TO UNCERTAINTIES IN THE DIFFERENCES IN 
THE MODEL TRACK SOLUTIONS, THERE IS LOW CONFIDENCE IN THE EXTENDED 
PORTION OF THE JTWC FORECAST TRACK.//
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