MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR 004// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (TAPAH), LOCATED APPROXIMATELY 213 NM SOUTH OF KADENA AB, OKINAWA, JAPAN HAS TRACKED NORTHWESTWARD AT 08 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) IMAGERY SHOWS CONVECTIVE BANDING WRAPPING INTO A PARTIALLY EXPOSED LOW LEVEL CIRCULATION (LLC) WITH ASSOCIATED CONVECTION PREDOMINANTLY LOCATED TO THE SOUTH AND SOUTHEAST. OVER THE LAST SIX HOURS, THE SYSTEM HAS STAGNATED IN DEVELOPMENT SHOWING NO SIGN OF STRUCTURAL IMPROVEMENT. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON EXTRAPOLATION FROM A 191307Z METOP-C ASCAT IMAGE AND THE PARTIALLY EXPOSED LLCC IN THE IR LOOP. THE INITIAL INTENSITY OF 40 KNOTS IS BASED ON PERSISTENCE FROM THE AFOREMENTIONED ASCAT IMAGE DUE TO THE APPARENT STAGNATION OF DEVELOPMENT. UPPER LEVEL ANALYSIS INDICATES THAT THE SYSTEM IS UNDER AN AREA OF LOW TO MODERATE (10-20 KTS) VERTICAL WIND SHEAR (VWS) AND IS SUPPORTED BY STRONG EQUATORWARD OUTFLOW AND HIGH (29-30 CELSIUS) SEA SURFACE TEMPERATURE (SST). TS 18W REMAINS LOCATED IN A COL BETWEEN A SUBTROPICAL RIDGE (STR) TO THE NORTHWEST AND A STR TO THE NORTHEAST. THE STR TO THE NORTHEAST HAS BEGUN TO BUILD AND IS TAKING OVER AS THE PRIMARY STEERING MECHANISMS FOR THE SYSTEM. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 18W WILL CONTINUE TO TRACK NORTHWESTWARD FOR THE NEXT 12 TO 24 HOURS AS IT BEGINS TO RECURVE AROUND THE STR TO THE NORTHEAST. THE SYSTEM WILL STEADILY INTENSIFY, REACHING A PEAK OF 60 KNOTS BY TAU 36 AS IT ROUNDS THE RIDGE AXIS. BY TAU 48, 18W WILL BE TRACKING TO THE NORTHEAST, PASSING BETWEEN CHEJU ISLAND AND SASEBO, JAPAN BEFORE TAU 72. AS TAPAH TRACKS THROUGH THE KOREAN STRAIT, LAND INTERACTION WILL BEGIN TO WEAKEN THE SYSTEM, A TREND THAT WILL PERSIST FOR THE DURATION OF THE FORECAST. ALSO, AROUND THIS TIME, INTERACTION WITH A PASSING MID-LATITUDE TROUGH WILL BEGIN THE EXTRATROPICAL TRANSITION. NUMERICAL GUIDANCE IS IN STRONG AGREEMENT THROUGH THIS PORTION OF THE FORECAST LENDING HIGH CONFIDENCE IN THE JTWC FORECAST TRACK. C. AFTER TAU 72, TS 18W BEGINS TO ENCOUNTER AN INCREASINGLY UNFAVORABLE ENVIRONMENT AS VWS INCREASES AND SSTS DECREASE. THE SYSTEM WILL CONTINUE TO WEAKEN AS IT TRACKS THROUGH THIS UNFAVORABLE ENVIRONMENT AND BECOMES EMBEDDED IN THE PASSING MID-LATITUDE TROUGH. TS 18W WILL MAKE LANDFALL AS AN EXTRATROPICAL CYCLONE OVER SOUTHERN HOKKAIDO AROUND TAU 96 BEFORE EXITING BACK INTO THE PACIFIC OCEAN. MODEL GUIDANCE REMAINS IN GOOD AGREEMENT WITH ONLY A 170NM SPREAD AT TAU 96, LENDING OVERALL HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN