WDPN31 PGTW 200300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W (THIRTEEN) WARNING
NR 02//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
TROPICAL STORM 13W (THIRTEEN), LOCATED APPROXIMATELY 423 NM EAST-
SOUTHEAST OF YOKOSUKA, JAPAN, HAS TRACKED NORTHWESTWARD AT 23 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) DEPICTS TIGHTLY CURVED BANDING WRAPPING INTO A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) EMBEDDED WITHIN THE
NORTHERN EXTENT OF A VERY BROAD MONSOON GYRE CIRCULATION. THE
INITIAL POSITION IS BASED WITH GOOD CONFIDENCE ON THE MSI AND A PAIR
OF RECENT ASCAT PASSES, WHICH REVEAL THE LLCC IS TRAILING BEHIND THE
DEEP CONVECTION DUE TO VERTICAL WIND SHEAR. THE INITIAL POSITION IS
ASSESSED AT 40 KNOTS, WHICH IS HIGHER THAN THE UNREPRESENTATIVE PGTW
DVORAK ESTIMATE OF 25 KNOTS. THE UPPER LEVEL ENVIRONMENT IS
PREDOMINANTLY FAVORABLE, WITH LOW TO MODERATE VERTICAL WIND SHEAR
(VWS) OVER THE LLCC, AND INCREASINGLY ROBUST POLEWARD OUTFLOW.
HOWEVER, CURRENT SEA SURFACE TEMPERATURES (SSTS) ARE MARGINALLY
SUPPORTIVE AT 26 DEGREES CELSIUS. TS 13W IS TRACKING NORTHWESTWARD
AROUND THE WESTERN PERIPHERY OF A SUBTROPICAL STEERING RIDGE (STR).
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TS 13W WILL CONTINUE TRACKING ALONG THE STR, AND WILL RECURVE
TO THE NORTHEAST BEGINNING AFTER TAU 12 IN RESPONSE TO AN
APPROACHING MID-LATITUDE TROUGH. BY TAU 12, SEA SURFACE TEMPERATURES
WILL FALL BELOW 26 DEGREES, AND TS 13W WILL BEGIN TO INTERACT WITH
THE BAROCLINIC TROUGH. EXTRATROPICAL TRANSITION (ETT) IS FORECAST TO
COMPLETE BY TAU 36 AS THE SYSTEM IS FULLY EMBEDDED WITHIN THE MID-
LATITUDE FLOW. THE INTENSITY IS EXPECTED TO REMAIN NEAR 40 KNOTS
OVER THE NEXT 12 HOURS AS SSTS REMAIN MARGINALLY SUPPORTIVE AND
OUTFLOW IS GOOD. A WEAKENING TREND WILL COMMENCE AFTER TAU 12 AND
LAST THROUGHOUT THE ETT PERIOD./=