MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (NAMTHEUN) WARNING NR 18// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 15W (NAMTHEUN), LOCATED APPROXIMATELY 5 NM EAST OF SASEBO, JAPAN, HAS TRACKED NORTHEASTWARD AT 04 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY AND THE KYUSHU RADAR LOOP SHOW THE CENTRAL CONVECTION HAS SIGNIFICANTLY ERODED AND DISPERSED AS THE SYSTEM MADE LANDFALL INTO WESTERN KYUSHU. THE INITIAL POSITION IS BASED ON THE RADAR LOOP AND FROM THE 18Z JMA OBSERVATION MAP WITH FAIR CONFIDENCE. THE INITIAL INTENSITY OF 35 KNOTS IS BASED ON THE RJTD DVORAK INTENSITY ESTIMATE AND INTERPOLATED FROM NEARBY SURFACE OBSERVATIONS. ENVIRONMENTAL CONDITIONS REMAIN UNFAVORABLE DUE TO HIGH (30-35 KNOT) VERTICAL WIND SHEAR (VWS) AND LAND INTERACTION. THE CYCLONE IS TRACKING NORTHEASTWARD ON THE POLEWARD SIDE OF THE SUBTROPICAL RIDGE AXIS. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS NAMTHEUN WILL ACCELERATE NORTHEASTWARD INTO THE SEA OF JAPAN (SOJ) WITH THE MID-LATITUDE WESTERLY FLOW WHERE IT WILL CONTINUE TO WEAKEN DUE TO COLD WATER (LESS THAN 25C) AND INCREASING VWS. CONCURRENTLY, TS 15W WILL UNDERGO EXTRA-TROPICAL TRANSITION (ETT) AND COMPLETE ETT AS A WEAK COLD-CORE LOW BY TAU 36. THERE IS A DISTINCT POSSIBILITY THAT THE SYSTEM WILL DISSIPATE IN THE SOJ DURING ETT. DYNAMIC MODEL GUIDANCE IS IN TIGHT AGREEMENT, LENDING HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN