WDPN32 PGTW 211500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (USAGI) WARNING NR 20// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 17W (USAGI), LOCATED APPROXIMATELY 315 NM EAST- SOUTHEAST OF HONG KONG, HAS TRACKED WEST-NORTHWESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS THE CONVECTIVE TOPS HAVE WARMED SLIGHTLY AS THE SYSTEM HAS NOW BECOME MORE ASYMMETRICAL AFTER DRAGGING ACROSS THE NARROW LUZON STRAIT. THE INITIAL POSITION IS BASED ON THE ABOVE ANIMATION WITH FAIR CONFIDENCE. THE INITIAL INTENSITY IS ON THE HIGH END OF DVORAK ESTIMATES FROM PGTW, RJTD, AND KNES. UPPER-LEVEL ANALYSIS INDICATES TY 17W REMAINS IN A FAVORABLE ENVIRONMENT WITH EXCELLENT RADIAL OUTFLOW AND LOW (05-10 KNOT) VERTICAL WIND SHEAR (VWS). THE CYCLONE IS TRACKING ALONG THE SOUTHERN PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO THE NORTH. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY USAGI IS FORECAST TO REMAIN ON A WEST-NORTHWESTWARD TRACK AS IT BEELINES TOWARDS HONG KONG THEN MAKES LANDFALL INTO SOUTHEASTERN CHINA. THE SYSTEM WILL GRADUALLY WEAKEN DUE TO SLIGHTLY INCREASING VWS BUT WILL STILL BE A STRONG TYPHOON WHEN IT IMPACTS HONG KONG AND CHINA. AFTER LANDFALL, 17W WILL RAPIDLY DECAY TOWARDS FULL DISSIPATION DUE TO THE FRICTIONAL EFFECTS OF THE RUGGED TERRAIN. NUMERIC MODEL GUIDANCE REMAINS IN TIGHT AGREEMENT, LENDING HIGH CONFIDENCE TO THE JTWC FORECAST TRACK. // NNNN