WDPN33 PGTW 122100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA) 
WARNING NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (NANGKA), LOCATED APPROXIMATELY 235 NM 
SOUTH-SOUTHWEST OF HONG KONG, HAS TRACKED WEST-NORTHWESTWARD AT 12 
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) 
SATELLITE IMAGERY SHOWS A SLOWLY CONSOLIDATING SYSTEM WITH 
FRAGMENTED RAIN BANDS FEEDING IN FROM THE SOUTH INTO A LOW LEVEL 
CIRCULATION THAT IS OBSCURED BY A SLIGHTLY-DEEPENED CENTRAL 
CONVECTION. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE 
BASED ON THE EIR AND COMPOSITE RADAR LOOPS. THE INITIAL INTENSITY OF 
40KTS IS HELD A TAD HIGHER THAN THE PGTW AND RJTD DVORAK INTENSITY 
ESTIMATES OF T2.5/35KTS TO REFLECT THE CONSOLIDATION. UPPER-LEVEL 
ANALYSIS INDICATES AN OVERALL FAVORABLE ENVIRONMENT WITH LOW 
VERTICAL WIND SHEAR AND GOOD EQUATORWARD OUTFLOW. ADDITIONALLY, SST 
(29-30C) VALUES IN THE SOUTH CHINA SEA ARE ALSO CONDUCIVE FOR 
DEVELOPMENT. TS 18W 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. THE STR WILL CONTINUE TO DRIVE TS NANGKA ON ITS CURRENT TRACK 
FOR THE ENTIRE FORECAST PERIOD, MAKING LANDFALL OVER HAINAN, CHINA, 
SHORTLY AFTER TAU 12, CROSS THE GULF OF TONKIN (GOT), THEN MAKE A 
FINAL LANDFALL OVER NORTHERN VIETNAM NEAR TAU 42. THE FAVORABLE 
ENVIRONMENT WILL FUEL A MODEST INTENSIFICATION TO 45KTS BY TAU 12 
JUST BEFORE LANDFALL INTO HAINAN. AFTERWARD, LAND INTERACTION WILL 
REDUCE THE INTENSITY TO 40KTS AS TS 18W EXITS INTO THE GOT. THE WARM 
SST IN THE GOT WILL PROMOTE A SECONDARY INTENSIFICATION TO 45KTS BY 
TAU 48 BEFORE LANDFALL INTO VIETNAM. AFTER LANDFALL, THE SYSTEM WILL 
RAPIDLY ERODE DUE TO FRICTIONAL EFFECTS FROM THE RUGGED TERRAIN, 
LEADING TO DISSIPATION BY TAU 72. NUMERICAL MODELS ARE IN TIGHT 
AGREEMENT WITH MINIMAL SPREADING AFTER TAU 36. THE JTWC FORECAST 
TRACK LIES NEAR THE MULTI-MODEL CONSENSUS WITH OVERALL HIGH 
CONFIDENCE.//
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