WDPN31 PGTW 261500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 24W (SON-TINH) WARNING 
NR 12//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 24W (SON-TINH), LOCATED APPROXIMATELY 355 NM 
EAST OF HUE, VIETNAM, HAS TRACKED WEST-NORTHWESTWARD AT 11 KNOTS 
OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY DEPICTS 
IMPROVED, TIGHTLY-CURVED CONVECTIVE BANDING WRAPPING INTO A POSSIBLE 
DEVELOPING, RAGGED EYE. A 261000Z SSMIS IMAGE DEPICTS SIGNIFICANTLY 
IMPROVED CONVECTIVE BANDING AND A WELL-DEFINED CENTER, THEREFORE, 
THERE IS GOOD CONFIDENCE IN THE INITIAL POSITION. EARLIER VISIBLE 
IMAGERY HAD SHOWN AN AREA OF STRATOCUMULUS (SC) OVER THE NORTHERN 
PERIPHERY OF THE LLCC AND NUMEROUS SURFACE OBSERVATIONS WITHIN THE 
SC FIELD INDICATED RELATIVELY COOLER, DRIER AIR WITH DEW POINTS 
RANGING FROM 20 TO 21C AND SLP VALUES NEAR 1011MB. ADDITIONALLY, AN 
UPPER-AIR SOUNDING FROM XISHA-DAO, APPROXIMATELY 170NM NORTHWEST OF 
THE CURRENT CENTER, SHOWED SOME DRY AIR IN THE MID-LEVELS, WHICH MAY 
BE HINDERING CONVECTIVE DEVELOPMENT OVER THE NORTHERN SEMI-CIRCLE. 
HOWEVER, THE TOTAL PRECIPITABLE WATER IMAGERY REVEALS A DEEP, MOIST 
ENVELOPE ASSOCIATED WITH THE SYSTEM. THE INITIAL INTENSITY OF 60 
KNOTS IS HEDGED SLIGHTLY HIGHER THAN DVORAK ESTIMATES OF 55 KNOTS 
FROM ALL AGENCIES BASED ON THE IMPROVED CONVECTIVE STRUCTURE AND 
INCIPIENT EYE. UPPER-LEVEL ANALYSIS CONTINUES TO DEPICT A 
FAVORABLE ENVIRONMENT WITH GOOD OUTFLOW AND WEAK VERTICAL WIND 
SHEAR. THE 26/00Z 500MB ANALYSIS DEPICTS AN EAST-WEST ORIENTED 
SUBTROPICAL RIDGE (STR) EXTENDING ALONG 22N EASTWARD TO TAIWAN WITH 
ZONAL WESTERLY FLOW NORTH OF THE STR.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING BULLETIN.
   B. TS 24W IS FORECAST TO CONTINUE TRACKING WEST-NORTHWESTWARD 
THROUGH THE FORECAST PERIOD UNDER THE STEERING INFLUENCE OF THE DEEP 
STR POSITIONED TO THE NORTH. THERE IS HIGH CONFIDENCE IN THE JTWC 
FORECAST TRACK BASED ON THE SOLID STEERING RIDGE AND GOOD AGREEMENT 
IN DYNAMIC MODEL GUIDANCE THROUGH TAU 36 WITH A 125NM SPREAD. AFTER 
TAU 36, MODEL GUIDANCE SHOWS INCREASING UNCERTAINTY AND IS SPLIT 
INTO TWO DISTINCT CLUSTERS. NOGAPS AND GFDN HAVE MAINTAINED A WEST-
NORTHWESTWARD TRACK INTO LAOS UNDER A STRONG SUBTROPICAL RIDGE. THE 
REMAINING GUIDANCE HAS SHIFTED TO MORE NORTHWESTWARD TRACK INTO 
NORTHERN VIETNAM OR THE GULF OF TONKIN WITH RAPID WEAKENING. TS 24W 
IS FORECAST TO INTENSIFY TO A PEAK INTENSITY OF 85 KNOTS BY TAU 24. 
ALTHOUGH TS 24W IS EXPECTED TO TRACK OVER MARGINAL OCEAN HEAT 
CONTENT, STRONG RADIAL OUTFLOW SHOULD OFFSET ANY MITIGATING FACTORS. 
BY TAU 36, TS 24W SHOULD GRADUALLY WEAKEN AS IT TRACKS ALONG THE 
COAST OF VIETNAM. THE SYSTEM SHOULD WEAKEN FURTHER AS IT MAKES 
LANDFALL AND IS FORECAST TO RAPIDLY WEAKEN OVER NORTHERN VIETNAM DUE 
TO FRICTIONAL EFFECTS.
C. IN THE EXTENDED TAUS, TS 24W IS FORECAST TO DISSIPATE BY TAU 96 
AND SLOW SIGNIFICANTLY OVER NORTHERN VIETNAM. MODEL GUIDANCE 
INDICATES INCREASING UPPER-LEVEL WESTERLY FLOW OVER NORTHERN 
VIETNAM, WHICH IS EXPECTED TO HALT THE WESTWARD PROGRESSION AND TURN 
THE SYSTEM EASTWARD. IN GENERAL, MODEL GUIDANCE INDICATES RAPID 
WEAKENING WITH THE REMNANTS TRACKING OVER THE GULF OF TONKIN.//
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