WDPN32 PGTW 210300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (USAGI) WARNING NR 18// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON 17W (USAGI), LOCATED APPROXIMATELY 271 NM SOUTH OF TAIPEI, TAIWAN, HAS TRACKED WEST-NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY REVEALS A SLIGHT WEAKENING TREND AS TY 17W CONTINUES TO GO THROUGH AN EYEWALL REPLACEMENT CYCLE (ERC) AND HAS VENTURED INTO THE THE LUZON STRAIGHT. ADDITIONALLY, THE LAND INTERACTION BETWEEN TAIWAN AND LUZON IS BEGINNING TO DISRUPT THE LOW-LEVEL INFLOW. A 202114Z SSMIS MICROWAVE IMAGE SHOWS THE CONTINUING ERC AND SLIGHT WEAKENING EVEN THOUGH TY 17W HAS MAINTAINED A 15 NM EYE, CONVECTION HAS BECOME MORE SHALLOW AND CENTRAL CONVECTION AROUND THE EYE HAS SIGNIFICANTLY DECREASED. THE INITIAL POSITION IS BASED UPON THE EYE SEEN IN THE AFOREMENTIONED SATELLITE IMAGERY WITH HIGH CONFIDENCE. THE INITIAL INTENSITY HAS BEEN LOWERED TO 120 KNOTS DUE TO THE WEAKENING TREND AND DVORAK CURRENT INTENSITY ESTIMATES THAT RANGE FROM 115 TO 140 KNOTS. UPPER-LEVEL ANALYSIS INDICATES TY 17W REMAINS IN A FAVORABLE ENVIRONMENT WITH RADIAL OUTFLOW AND LOW (05-10 KNOT) VERTICAL WIND SHEAR FROM A POINT SOURCE ANTICYCLONE THAT HAS REMAINED OVER THE SYSTEM. TY 17W CONTINUES TRACKING GENERALLY WEST ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) THAT EXTENDS FROM CENTRAL CHINA INTO THE EAST CHINA SEA. 3. FORECAST REASONING. A. NO CHANGES TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 17W IS FORECAST TO REMAIN ON A WEST-NORTHWEST TRACK THROUGH TAU 48 AS THE SYSTEM TRACKS UNDER THE STEERING INFLUENCE OF THE EAST- WEST ORIENTED STR, MAKING LANDFALL AFTER TAU 36 NEAR HONG KONG. TY 17W IS FORECAST TO MAINTAIN ITS CURRENT INTENSITY THROUGH THE NEXT 12 HOURS WITH A POSSIBILITY OF RE-STRENGTHENING AS TY 17W PASSES THROUGH THE LUZON STRAIGHT AND INTO THE MORE OPEN WATERS OF THE SOUTH CHINA SEA. AFTER WHICH, SLOW WEAKENING IS EXPECTED PRIOR TO LANDFALL NEAR HONG KONG. THE FRICTIONAL EFFECTS OF LAND AFTER LANDFALL WILL LEAD TO A GENERAL WEAKENING TREND AS THE SYSTEM TRACKS JUST INLAND OF THE COAST. C. TY 17W SHOULD ERODE QUICKLY AS IT TRACKS ALONG THE SOUTHERN CHINA COAST AND INTO NORTHERN VIETNAM. FULL DISSIPATION IS EXPECTED BY TAU 96 DUE TO THE FRICTIONAL EFFECTS OF LAND. NUMERIC MODEL GUIDANCE REMAINS IN TIGHT AGREEMENT THROUGH THE FORECAST PERIOD, THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN