WDPN31 PGTW 210300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (JELAWAT) WARNING 
NR 03//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (JELAWAT), LOCATED APPROXIMATELY 575 NM 
EAST OF MANILA, PHILIPPINES, HAS TRACKED WEST-SOUTHWESTWARD AT 07 
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE 
IMAGERY (MSI) SHOWS CENTRAL CONVECTION CONSOLIDATING AROUND THE LOW 
LEVEL CIRCULATION CENTER (LLCC) WITH AN OUTER BAND OF DEEP 
CONVECTION WRAPPING AROUND THE SYSTEM'S WESTERN AND SOUTHERN 
PERIPHERY. A 202320Z SSMIS 91GHZ IMAGE DEPICTS TIGHTLY CURVED 
BANDING WRAPPING INTO THE LLCC, WITH THE DEEPEST CONVECTION IN THE 
WESTERN SEMI-CIRCLE. DESPITE FAVORABLE CONDITIONS OVERALL, 18W HAS 
STRUGGLED TO MAINTAIN ITS ORGANIZATION. THE INITIAL POSITION IS 
BASED ON ANIMATED MSI AND AN EXTRAPOLATION FROM A 202129Z WINDSAT 
37GHZ IMAGE. THE INITIAL INTENSITY IS ASSESSED AT 35 KTS, IN 
AGREEMENT WITH DVORAK ESTIMATES FROM PGTW, KNES AND RJTD. UPPER-
LEVEL ANALYSIS INDICATES THE SYSTEM IS IN A FAIR ENVIRONMENT, WITH 
LOW (05-10 KNOT) VERTICAL WIND SHEAR (VWS) AND GOOD EQUATORWARD 
OUTFLOW APPARENT IN ANIMATED WATER VAPOR IMAGERY. A LARGE AREA OF 
DEEP SUBTROPICAL RIDGING TO THE NORTHEAST IS CONSTRAINING POLEWARD 
OUTFLOW TO A SMALL RIBBON AND AN APPROACHING MID-LATITUDE TROUGH HAS 
PROVIDED LITTLE ADDITIONAL OUTFLOW. THE CYCLONE IS TRACKING ALONG 
THE SOUTHWESTERN EXTENSION OF THE SUBTROPICAL RIDGE (STR) ANCHORED 
EAST OF JAPAN. WARM (28-30 DEGREE) SEA SURFACE TEMPERATURES (SSTS) 
EXIST THROUGHOUT THE PHILIPPINE SEA. 
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. THE EASTWARD MOVING MID-LATITUDE TROUGH MOVING OVER THE KOREAN 
PENINSULA WILL CONTINUE TO WEAKEN THE STEERING STR, CAUSING TS 18W 
TO SLOW DOWN AND BECOME QUASI-STATIONARY. WITH LOW VWS, SUSTAINED 
EQUATORWARD OUTFLOW, AND FAVORABLE SSTS, THE SYSTEM IS FORECAST TO 
STEADILY INTENSIFY INTO A TYPHOON PRIOR TO TAU 48. BETWEEN TAUS 36-
48, THE STR WILL REBUILD AND START TO TURN 18W TO THE NORTHWEST. 
THERE IS A LARGE SPREAD IN THE NUMERICAL MODEL GUIDANCE AS THE 
SYSTEM ENTERS A WEAK STEERING ENVIRONMENT. SOME OF THE SPREAD IS 
ATTRIBUTED TO DIFFERENCES IN SPEED, WHILE SOME MODELS APPEAR TO OVER-
ANALYZE THE IMPACT OF THE MID-LATITUDE TROUGH AND TURN THE SYSTEM TO 
THE NORTH AS A RESULT. DUE TO THIS LARGE MODEL SPREAD AND 
UNCERTAINTIES ASSOCIATED WITH QUASI-STATIONARY MOTION, THERE IS LOW 
CONFIDENCE IN THIS PORTION OF THE FORECAST.
   C. DURING THE EXTENDED TAUS, THE REBUILDING STR TO THE NORTHEAST 
WILL STEER THE SYSTEM IN A NORTHWESTWARD DIRECTION AS THE PRIMARY 
STEERING MECHANISM GRADUALLY TRANSITIONS TO ANOTHER STR OVER EASTERN 
CHINA. THIS CHANGE IN STEERING MECHANISMS MAY EVENTUALLY RESULT IN A 
MORE WESTWARD TRACK MOTION BEYOND THE FORECAST PERIOD. 
GRADUAL INTENSIFICATION SHOULD CONTINUE THROUGHOUT THE FORECAST 
PERIOD, REACHING 95 KTS AT TAU 120. THE NUMERICAL GUIDANCE AT TAU 
120 HAS A SPREAD OF OVER 600 NM, HOWEVER, THE CONSENSUS HAS REMAINED 
FAIRLY CONSISTENT, WITH A TRACK TOWARD THE LUZON STRAIT. THE 
FORECAST IS SLIGHTLY SLOWER AND WEST OF CONSENSUS TO ACCOUNT FOR 
EGRR WHICH ERRONEOUSLY TURNS THE SYSTEM NORTH INTO THE RIDGE. DUE TO 
THE LARGE MODEL SPREAD, THERE IS LOW CONFIDENCE IN THIS PORTION OF 
THE FORECAST.//
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