Tropical Storm TAPAH Advisory Fri Sep 20

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN)
WARNING NR 002//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 18W (EIGHTEEN), LOCATED APPROXIMATELY 256
NM SOUTH OF KADENA AB, OKINAWA, JAPAN HAS TRACKED WESTWARD AT 01
KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY SHOWS AN EXPOSED LOW LEVEL CIRCULATION (LLC) WITH CONVECTION
LOCATED TO THE NORTHEAST AND SOME SHEARED TO THE SOUTHWEST. DUE TO
THE WELL-DEFINED LLC, THERE IS HIGH CONFIDENCE IN THE INITIAL
POSITION. THE INITIAL INTENSITY OF 30 KTS IS BASED ON MULTI-AGENCY
DVORAK ESTIMATES OF T1.5 (PGTW) AND T2.0 (RJTD) AS WELL AS A 190049Z
METOP-B ASCAT PASS DISPLAYING SMALL AREAS OF 30-35 KT WINDS AROUND
THE LLC. UPPER LEVEL ANALYSIS INDICATES THAT THE SYSTEM REMAINS SPLIT
UNDER STRONG (25-30KT) NORTHEASTERLY VERTICAL WIND SHEAR (VWS) TO THE
EAST AND LIGHT (10-15KT) VWS ON THE WESTERN PERIPHERY OF THE SYSTEM.
HOWEVER, STRONG EQUATORWARD OUTFLOW OFFSETS THE VWS. SEA SURFACE
TEMPERATURE (SST) REMAINS HIGH AT 29-30C. TD 18W IS IN A COL BETWEEN
THE SUBTROPICAL RIDGE (STR) TO THE NORTHWEST AND THE STR TO THE
NORTHEAST AND REMAINS QUASI-STATIONARY.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. AS THE STR BECOMES THE PREDOMINANT STEERING MECHANISM, TD 18W
WILL BEGIN TO SLOWLY TRACK NORTHWESTWARD. BETWEEN TAU 24 AND 36, IT
WILL SHIFT TO A MORE POLEWARD TRACK AS IT ROUNDS THE STR AXIS. AFTER
CRESTING THE STR, TD 18W WILL TRACK MORE NORTH-NORTHEASTWARD ON THE
POLEWARD SIDE OF THE STR. THE VWS WILL BECOME MORE FAVORABLE AS THE
SYSTEM TRACKS POLEWARD, ENHANCING GRADUAL INTENSIFICATION TO A PEAK
OF 55 KTS BY TAU 48, AIDED BY ENHANCED POLEWARD OUTFLOW ASSOCIATED
WITH THE PREVAILING WESTERLIES. BY TAU 72, THE CYCLONE WILL BE JUST
EAST OF CHEJU ISLAND, SOUTH KOREA, BEGINNING TO INTERACT WITH LAND.
MODEL GUIDANCE HAS CONTINUED TO COME INTO BETTER AGREEMENT AS
CROSS-TRACK VARIATION DECREASES TO A SPREAD OF 160 NM BY TAU 72.
HOWEVER, THERE REMAINS LOW CONFIDENCE IN THIS PORTION OF THE JTWC
FORECAST DUE TO THE UNCERTAINTY IN THE INITIAL STORM MOTION AND
SPEED.
   C. AFTER TAU 72, TD 18W WILL ENTER THE SEA OF JAPAN (SOJ) AND
ACCELERATE NORTHEASTWARD, RETURNING BACK INTO THE PACIFIC OCEAN JUST
AFTER TRANSITING ACROSS HOKKAIDO, JAPAN, BY TAU 120. TD 18W WILL
GRADUALLY ERODE AS IT TRANSITS THROUGH THE SOJ AS IT ENCOUNTERS
INCREASING VWS AND COOLING SST. AS THIS OCCURS, THE SYSTEM WILL ALSO
UNDERGO EXTRA-TROPICAL TRANSITION (BEGINNING BETWEEN TAU 72 AND 96)
AS IT BECOMES EMBEDDED IN THE BAROCLINIC ZONE, COMPLETING
EXTRA-TROPICAL TRANSITION BY TAU 120. NUMERICAL MODELS ARE IN GENERAL
AGREEMENT BUT SPREADS OUT TO OVER 400 NM BY TAU 120, LENDING LOW
CONFIDENCE IN THE EXTENDED PORTION OF THE JTWC TRACK FORECAST.//
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