Tropical Storm TAPAH Advisory Fri Sep 20

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR
007//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 18W (TAPAH), LOCATED APPROXIMATELY 147 NM SOUTH OF
KADENA AB, OKINAWA, JAPAN HAS TRACKED NORTH-NORTHEASTWARD AT 11 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) WHICH APPEARS TO HAVE BECOME MORE
SYMMETRICAL OVER THE PAST SIX HOURS. THE INITIAL POSITION IS PLACED
WITH FAIR CONFIDENCE BASED ON A 200945Z F-17 SSMIS 91 GHZ MICROWAVE
IMAGE DEPICTING A LLC WITH DEEP CONVECTION EXTENDING TO THE SOUTH AND
SOUTHEAST. EIR REVEALS THAT THIS CONVECTION IS WRAPPING INTO THE
NORTHERN AND EASTERN QUADRANTS OF THE SYSTEM. THE INITIAL INTENSITY
OF 50 KNOTS IS BASED ON DVORAK ESTIMATES OF T3.0 (50 KTS) BY RJTD AND
T3.5 (55 KTS) BY PGTW. DUE TO THE BROAD NATURE OF THE SYSTEM, DVORAK
ESTIMATES HAVE BEEN CONSISTENTLY LOW THROUGHOUT THE PREVIOUS FORECAST
PERIODS, LENDING GOOD CONFIDENCE TO THE INITIAL INTENSITY ESTIMATE.
UPPER LEVEL ANALYSIS INDICATES THAT TS 18W HAS MOVED INTO A REGION OF
LOW TO MODERATE (10-15 KTS) VERTICAL WIND SHEAR (VWS) WITH MODERATE
EQUATORWARD OUTFLOW. THE WARM (28-29 CELSIUS) SEA SURFACE TEMPERATURE
(SST) REMAINS CONDUCIVE FOR INTENSIFICATION AS TS 18W TRACKS NORTH-
NORTHWESTWARD ALONG THE PERIPHERY OF A SUBTROPICAL RIDGE (STR)
LOCATED TO THE NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. FOLLOWING ITS PRIMARY STEERING MECHANISM, TS 18W WILL TRACK
NORTH-NORTHWESTWARD ALONG THE STR FOR THE NEXT 24 HOURS. ALONG THIS
ROUTE, TS 18W WILL INTENSIFY TO ITS PEAK INTENSITY (65 KTS) BY TAU
36. BETWEEN TAU 30 AND 36, A MID-LATITUDE TROUGH, CURRENTLY LOCATED
TO THE NORTHWEST, WILL MOVE NORTH OF THE SYSTEM AND BEGIN TO ENHANCE
POLEWARD OUTFLOW PRIOR TO THE SYSTEM BECOMING EMBEDDED IN THE TROUGH.
CONCURRENTLY, TS 18W WILL ROUND THE STR AXIS AND BEGIN A NORTHEAST
TRACK TOWARDS THE SEA OF JAPAN (SOJ). AT TAU 48, VWS WILL INCREASE
AND SST WILL BEGIN TO COOL, SLOWLY ERODING THE SYSTEM AS IT INTERACTS
WITH LAND UPON TRANSITING INTO THE SOJ. BEFORE TAU 72, TS 18W WILL
BEGIN EXTRATROPICAL TRANSITION (ETT) AS IT ACCELERATES ALONG A
NORTHEAST TRACK. NUMERICAL GUIDANCE IS IN GOOD AGREEMENT THROUGH THIS
PORTION OF THE FORECAST, SUPPORTING HIGH CONFIDENCE IN THE JTWC
FORECAST TRACK.
   C. AFTER TAU 72, TS 18W WILL CONTINUE TO WEAKEN DUE TO HIGH VWS
VALUES AS IT BECOMES EMBEDDED IN THE MID-LATITUDE TROUGH. PRIOR TO
MAKING LANDFALL AT HOKKAIDO, TS 18W WILL TRANSITION INTO AN
EXTRATROPICAL CYCLONE. THE SYSTEM WILL RETURN TO THE PACIFIC OCEAN BY
TAU 84 AND CONTINUE ON A NORTHEASTERLY COURSE FOR THE REMAINDER OF
THE FORECAST. MODEL GUIDANCE IS IN GOOD AGREEMENT FOR THE EXTENDED
PORTION OF THE FORECAST, LENDING TO HIGH CONFIDENCE IN THE JTWC
FORECAST TRACK.//
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