MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (ETAU) WARNING NR 09// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (ETAU), LOCATED APPROXIMATELY 147 NM SOUTHWEST OF YOKOSUKA, JAPAN, HAS TRACKED NORTHWARD AT 21 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS A BROAD AND ELONGATED SYSTEM THAT RECENTLY UNDERWENT A FLARE UP IN CONVECTION JUST PRIOR TO INTERACTING WITH THE JAPANESE COASTLINE. HOWEVER, OVER THE PAST TWO HOURS THAT CONVECTION HAS STARTED TO DISSIPATE. IN ADDITION, WATER VAPOR IMAGERY SHOWS THAT THE SYSTEM IS BEGINNING TO INTERACT WITH THE BOUNDARY LAYER AND STARTING EXTRA-TROPICAL TRANSITION (ETT). AN 081752Z SSMI 37GHZ MICROWAVE IMAGE SHOWS TIGHT CURVED BANDING AT THE SURFACE WHICH, IN CONJUNCTION WITH RJTD RADAR FIXES, SUPPORTS THE CURRENT POSITION WITH GOOD CONFIDENCE. THE INITIAL INTENSITY IS ASSESSED AT 55 KNOTS BASED ON AN OLDER ASCAT PASS AND CONCURRING DVORAK INTENSITY ESTIMATES OF T3.5 (55 KNOTS) FROM ALL AGENCIES. UPPER LEVEL ANALYSIS INDICATES THAT TS 18W IS BEGINNING TO EMBED IN THE BAROCLINIC ZONE WITH AN UPPER LEVEL TROUGH ENGULFING THE SYSTEM AND PRODUCING HIGH (30-40 KNOT) SOUTHWESTERLY VERTICAL WIND SHEAR (VWS). TS ETAU IS TRACKING BETWEEN THE WESTERN EXTENSION OF A DEEP-LAYERED SUB- TROPICAL RIDGE (STR) TO THE NORTHEAST AND THE AFOREMENTIONED TROUGH. 3. FORECAST REASONING. A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE PREVIOUS PROGNOSTIC REASONING MESSAGE; HOWEVER, WIND RADII HAVE BEEN ENLARGED BASED ON AN 081130Z ASCAT PASS. B. TS 18W WILL CONTINUE TO TRACK TO THE NORTHWEST UNDER THE INFLUENCE OF THE TIGHTENED GRADIENT BETWEEN THE STR AND THE UPPER LEVEL TROUGH. THE SYSTEM WILL FURTHER WEAKEN DUE TO LAND INTERACTION AS IT CROSSES OVER MAINLAND JAPAN. IN ADDITION, TS ETAU WILL EMBED FURTHER INTO THE BAROCLINIC ZONE AND CONTINUE TO GAIN EXTRA-TROPICAL CHARACTERISTICS. TS 18W WILL COMPLETE ETT BY THE END OF THE FORECAST PERIOD OR POSSIBLY SOONER AS A WEAK COLD-CORE LOW. DYNAMIC MODEL GUIDANCE IS IN GOOD OVERALL AGREEMENT THROUGH THE FORECAST PERIOD, WHICH LEND TO HIGH CONFIDENCE IN THE JTWC FORECAST.// NNNN NNNN