WDPN33 PGTW 270300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING NR 015// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 34.9N 142.5E INITIAL INTENSITY: 45 KTS GEOGRAPHIC REFERENCE: 140 NM EAST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 03 KTS SIGNIFICANT WAVE HEIGHT: 20 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 11W (NEPARTAK) REMAINS ASSESSED AS A SUBTROPICAL STORM. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS SHALLOW BANDING WRAPPING TIGHTLY INTO A FULLY-EXPOSED LOW-LEVEL CIRCULATION CENTER, WHICH SUPPORTS THE INITIAL POSITION WITH HIGH CONFIDENCE. ANIMATED WATER VAPOR IMAGERY SHOWS THAT NEPARTAK IS CENTERED BENEATH A STRONG UPPER-LEVEL LOW WITH DRY AIR ADVECTING AROUND THE SYSTEM. AFTER AN UNEXPECTED, SHORT-LIVED WEST-SOUTHWESTWARD TRACK, 11W HAS ACCELERATED POLEWARD ALONG THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE. ALONG WITH HOURLY RADAR FIXES FROM RJTD, A 262323Z ASCAT-A BULLSEYE IMAGE ALSO SUPPORTS THE INITIAL POSITION AND INITIAL INTENSITY OF 45 KTS WITH HIGH CONFIDENCE. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: LOW TO MID-LEVEL SUBTROPICAL RIDGE (STR) TO THE EAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: ST2.5 - 35-40 KTS RJTD: T2.0 - 30 KTS CIMSS SATCON: 48 KTS AT 262125Z CIMSS ADT: 39 KTS AT 262340Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE VWS: 5-10 KTS SST: 27-28 CELSIUS OUTFLOW: WEAK WESTWARD ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: 11W IS FORECAST TO TRACK NORTH-NORTHWESTWARD THROUGH TAU 24 AS IT MAKES LANDFALL AND TRACKS OVER THE JAPAN ALPS. SLIGHT WEAKENING IS EXPECTED AS THE SUBTROPICAL STORM TRACKS OVERLAND. AFTER TAU 24, 11W WILL TURN NORTHWESTWARD TO WEST- NORTHWESTWARD AS THE STEERING TRANSITIONS TO AN EXTENSION OF THE STR POSITIONED TO THE NORTH. AS 11W REEMERGES OVER THE EAST SEA, THE SYSTEM WILL WEAKEN STEADILY DUE TO INCREASING VERTICAL WIND SHEAR (25 TO 35 KNOTS) ASSOCIATED WITH STRONG NORTHERLY FLOW ALOFT WITH DISSIPATION EXPECTED BY TAU 72. MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE SUPPORTS THE JTWC TRACK FORECAST WITH HIGH CONFIDENCE AND IS POSITIONED CLOSE TO THE MULTI-MODEL CONSENSUS. THE TRACK FORECAST IS ALSO SUPPORTED BY THE 261200Z ECMWF ENSEMBLE STRIKE PROBABILITY PRODUCT. THE JTWC INTENSITY FORECAST IS SUPPORTED BY THE INTENSITY CONSENSUS (ICNW) WITH HIGH CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH INTENSITY 0 - 72 HR: HIGH// NNNN