WDPN31 PGTW 221500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (JELAWAT) WARNING 
NR 09//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (JELAWAT), LOCATED APPROXIMATELY 545 NM 
EAST-SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WESTWARD AT 03 
KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY 
DEPICTS A SLOWLY CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC) 
WITH A 130NM DIAMETER CENTRAL DENSE OVERCAST (CDO) AND MULTIPLE 
CONVECTIVE BANDS WRAPPING INTO THE SYSTEM. A 220848Z SSMIS 37GHZ 
IMAGE INDICATES TIGHTLY-CURVED CONVECTIVE BANDING WRAPPING INTO A 
WELL-DEFINED CENTER, THEREFORE, THERE IS INCREASED CONFIDENCE IN THE 
INITIAL POSITION AND RECENT MOTION WESTWARD. ANIMATED WATER VAPOR 
IMAGERY INDICATES THAT THE LLCC IS LOCATED JUST SOUTHWEST OF AN 
UPPER-LEVEL ANTICYCLONE, WHICH IS PRODUCING WEAK, NORTHEASTERLY 
VERTICAL WIND SHEAR (VWS) AS WELL AS STRONG DIFFLUENCE; HOWEVER, VWS 
APPEARS TO HAVE ABATED SLIGHTLY AND POLEWARD OUTFLOW HAS IMPROVED 
DUE TO A DEVELOPING TUTT CELL NEAR 15N 137E. THE INITIAL INTENSITY 
IS ASSESSED AT 55 KNOTS BASED ON THE IMPROVED ORGANIZATION AND 
DVORAK ESTIMATES OF 55 KNOTS FROM BOTH KNES, RJTD AND PGTW. TS 18W 
IS CURRENTLY LOCATED WITHIN A WEAK STEERING ENVIRONMENT. THE 22/00Z 
500MB ANALYSIS DEPICTS A LARGE BREAK BETWEEN THE WESTERN SUBTROPICAL 
RIDGE (STR) AND THE EASTERN STR DUE TO A BROAD TROUGH POSITIONED 
OVER ASIA. THE WESTERN STR EXTENDS FROM HAINAN ISLAND EASTWARD ALONG 
20N TO ABOUT 130E WHILE THE EASTERN STR IS POSITIONED EAST OF 140E.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. TS 18W IS FORECAST TO DRIFT SLOWLY WEST-NORTHWESTWARD THROUGH 
TAU 24 UNDER THE STEERING INFLUENCE OF THE WEAK WESTERN STR 
POSITIONED NORTH OF THE SYSTEM. THIS STEERING INFLUENCE IS EXPECTED 
TO BE SHORT-LIVED AS A MIDLATITUDE SHORTWAVE TROUGH DEEPENS OVER THE 
EAST CHINA SEA IN THE NEXT 12 HOURS. AFTER THIS TROUGH PROPAGATES 
EASTWARD, THE EASTERN STR IS FORECAST TO BUILD NORTHEAST OF 18W 
DRIVING THE SYSTEM NORTHWESTWARD; HOWEVER, TRACK SPEEDS ARE STILL 
EXPECTED TO BE SLOW (04-05 KNOTS). WITH THE EXCEPTION OF THE JGSM 
MODEL, DYNAMIC MODEL GUIDANCE IS IN BETTER AGREEMENT WITH A 135NM 
SPREAD AT TAU 72; HOWEVER, GFDN, NOGAPS, JGSM AND UKMO HAVE BEEN 
ERRATIC. BASED ON THE UPPER-LEVEL ANALYSIS, THE JTWC FORECAST 
CONTINUES TO FAVOR THE WESTWARD TO NORTHWESTWARD TRACK, AS DEPICTED 
BY THE ECMWF MODEL, OVER THE UNLIKELY NORTHWARD TRACK INTO THE STR 
(MOST NOTABLY JGSM, UKMO, WBAR). DUE TO THE WEAK STEERING 
ENVIRONMENT, THERE IS LOW CONFIDENCE IN THE FORECAST TRACK.
   C. IN THE EXTENDED TAUS, TS 18W IS FORECAST TO TURN INCREASINGLY 
WEST-NORTHWESTWARD AS IT TRANSITIONS TO THE STEERING INFLUENCE OF 
THE WESTERN STR. DYNAMIC MODEL GUIDANCE CONTINUES TO DIVERGE WITH A 
LARGE SPREAD IN SOLUTIONS, THERFORE, THERE IS LOW CONFIDENCE IN THE 
EXTENDED FORECAST. TS 18W SHOULD CONTINUE TO INTENSIFY THROUGH THE 
PERIOD AND IS EXPECTED TO REACH A PEAK INTENSITY OF 115 KNOTS NEAR 
TAU 72.//
NNNN