WDPN33 PGTW 131500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA) WARNING NR 008// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (NANGKA), LOCATED APPROXIMATELY 286 NM EAST-SOUTHEAST OF HANOI, VIETNAM, HAS TRACKED WEST NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS A STRENGTHENED SYSTEM, WITH CONVECTION HAVING CONSOLIDATED AND DEEPENED NEAR THE ASSESSED LLCC OVER THE PAST FEW HOURS. RADAR IMAGERY DEPICTS THAT THE SYSTEM MADE LANDFALL VERY NEAR THE 131200Z HOUR JUST SOUTH OF QIONGHAI, CHINA. THE RADAR IMAGERY ALSO DEPICTS A VERY CLEAR, ALMOST EYE LIKE FEATURE AS THE SYSTEM MADE LANDFALL, WHICH WHEN COMBINED WITH A 131040Z SSMIS 36 GHZ MICROWAVE IMAGE, PROVIDED HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY HAS BEEN UPGRADED TO 50 KNOTS WITH MODERATE CONFIDENCE, HEDGED UPWARD FROM DVORAK CURRENT INTENSITY ESTIMATES OF T2.5 FROM BOTH PGTW AND RJTD, IN LIGHT OF AN ADVANCED DVORAK TECHNIQUE ESTIMATE OF T3.3 (51 KNOTS) AND A SATELLITE CONSENSUS ESTIMATE OF 51 KNOTS AT 0600Z. COMBINING THE AUTOMATED ANALYZES WITH THE OVERALL STRUCTURE EVIDENT IN THE RADAR LEADS TO THE HIGHER INTENSITY ESTIMATE. ENVIRONMENTAL ANALYSIS INDICATES THE SYSTEM LIES IN A GENERALLY FAVORABLE ENVIRONMENT WITH LOW (10-15KT) VERTICAL WIND SHEAR, AND GOOD DIVERGENT WESTWARD OUTFLOW. THE CYCLONE IS TRACKING ALONG THE SOUTHWESTERN 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. TS 18W HAS MADE LANDFALL ALONG THE EAST COAST OF HAINAN ISLAND AS PREVIOUSLY DISCUSSED. THE TRACK OVER THE PAST SIX HOURS HAS BEEN SLIGHTLY MORE NORTH THAN PREVIOUSLY FORECAST IN RESPONSE TO DEVELOPMENT OF A SMALL WEAKNESS AND SHIFT IN ORIENTATION IN THE STR. THE FORECAST TRACK IS EXPECTED TO FLATTEN TO MORE WESTWARD OVER THE NEXT 12 TO 24 HOURS AS THE STR BECOMES MORE EAST-WEST ORIENTED, AND THE LLCC IS FORECAST TO REEMERGE OVER THE SOUTHERN GULF OF TONKIN BY TAU 12. A CLOSE TO DUE WESTWARD TRACK IS FORECAST THEREAFTER THROUGH LANDFALL ALONG THE VIETNAMESE COASTLINE AROUND TAU 30. NOW THAT THE SYSTEM IS OVER HAINAN, A SHORT-TERM WEAKENING IS EXPECTED DUE PRIMARILY TO TERRAIN INTERACTION. ONCE THE SYSTEM AGAIN MOVES OVER WATER, ANOTHER ROUND OF INTENSIFICATION WILL OCCUR AS THE SYSTEM TAPS INTO DIVERGENT WESTWARD OUTFLOW AND TRANSITS VERY WARM (NEAR 30 DEG CELSIUS) WATERS WITH LOW VWS, WITH A FORECASTED PEAK OF 55 KNOTS AT TAU 24. AFTER MAKING LANDFALL OVER NORTHERN VIETNAM, THE SYSTEM WILL RAPIDLY WEAKEN OVER THE MOUNTAINOUS TERRAIN OF VIETNAM AND WILL DISSIPATE AS A TROPICAL CYCLONE OVER NORTHERN LAOS BY TAU 72. NUMERICAL MODELS REMAIN IN GOOD AGREEMENT THOUGH THE ENTIRE FORECAST PERIOD WITH ONLY MINIMAL SPREAD AFTER LANDFALL IN VIETNAM. THE JTWC FORECAST TRACK LIES ON THE NORTHERN SIDE OF THE GUIDANCE ENVELOPE THROUGH THE FORECAST PERIOD WITH HIGH CONFIDENCE.// NNNN