Tropical Storm TAPAH Advisory Cts, 21.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR
008//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (TAPAH), LOCATED APPROXIMATELY 113 NM
SOUTHWEST OF KADENA AB, HAS TRACKED NORTH-NORTHWESTWARD AT 13 KNOTS
(KTS) OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR)
SATELLITE IMAGERY SHOWS CONVECTIVE BANDING WRAPPING INTO A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION (LLC) WITH THE BULK OF THE DEEP
CONVECTION IN THE EASTERN HALF OF THE SYSTEM. THERE IS GOOD
CONFIDENCE IN THE INITIAL POSITION BASED ON THE EIR, A 201726Z AMSR2
89 GHZ MICROWAVE IMAGE, AND IT CLOSELY ALIGNS WITH RJTD RADAR FIXES.
THE INITIAL INTENSITY OF 55 KTS ALIGNS WITH THE PGTW DVORAK FIX
INTENSITY OF T3.5 (55 KTS) AND IS SUPPORTED BY OBSERVATIONS UP TO 55
KTS FROM OBSERVATIONS ON OKINAWA. LOW (10-15 KT) VERTICAL WIND SHEAR
(VWS), GOOD EQUATORWARD AND POLEWARD OUTFLOW, AND WARM (27-28
CELSIUS) SEA SURFACE TEMPERATURE (SST) MAKE THE ENVIRONMENT
SUPPORTIVE OF INTENSIFICATION. TS 18W IS TRACKING POLEWARD ALONG THE
WESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. BY TAU 36, TS 18W WILL HAVE ROUNDED THE STR AXIS AND BEGIN
TURNING NORTHEASTWARD. THERE IS A BRIEF PERIOD WHERE THE ENVIRONMENT
WILL SUPPORT SOME INTENSIFICATION BEFORE STRONG VWS, LAND
INTERACTION, AND COOL SST WILL CAUSE GRADUAL WEAKENING. THE
INTENSITY IS EXPECTED TO PEAK AT 65 KTS AS 18W APPROACHES THE
CHANNEL BETWEEN SOUTH KOREA AND JAPAN AROUND TAU 36. BY TAU 48, TS
18W WILL MOVE INTO MID-LATITUDE BAROCLINIC ZONE AND BEGIN
EXTRATROPICAL TRANSITION (ETT). ETT IS EXPECTED TO COMPLETE BY TAU
72 AS THE SYSTEM TRANSITIONS INTO A GALE-FORCE COLD CORE LOW.
NUMERICAL GUIDANCE IS IN STRONG AGREEMENT THROUGH TAU 48. AFTER TAU
48, ALONG-TRACK SPEED DIFFERENCES INCREASE THE SPREAD AMONG THE
MODELS SLIGHTLY. BASED ON STRONG AGREEMENT IN THE FORECAST, THERE IS
GOOD CONFIDENCE IN THE JTWC TRACK FORECAST.//
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