Tropical Storm JELAWAT Advisory Fri Sep 21

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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