WDPN33 PGTW 130300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA) 
WARNING NR 006//
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 MULTISPECTRAL SATELLITE 
IMAGERY SHOWS THE SYSTEM HAS MAINTAINED ITS OVERALL CONVECTIVE 
SIGNATURE WITH LONG FRAGMENTED RAIN BANDS FEEDING IN MOSTLY FROM THE 
SOUTH INTO A LOW LEVEL CIRCULATION (LLC) THAT IS OBSCURED BY THE 
CENTRAL CONVECTION. THE INITIAL POSITION IS PLACED WITH FAIR 
CONFIDENCE BASED ON THE ABOVE ANIMATION AND ON A LLC FEATURE IN THE 
122327Z SSMIS MICROWAVE PASS. THE INITIAL INTENSITY OF 40KTS IS HELD 
A BIT HIGHER THAN THE PGTW AND RJTD DVORAK INTENSITY ESTIMATES OF 
T2.5/35KTS TO REFLECT THE SUSTAINED CONVECTIVE STRUCTURE. ANALYSIS 
INDICATES A FAVORABLE ENVIRONMENT WITH LOW (10-15KT) VERTICAL WIND 
SHEAR, GOOD EQUATORWARD OUTFLOW, AND WARM (29-30C) SST IN THE SOUTH 
CHINA SEA. 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. THE STR WILL REMAIN THE PRIMARY STEERING MECHANISM FOR TS 
NANGKA AS IT DRIVES IT ON ITS CURRENT WEST-NORTHWESTWARD TRACK FOR 
THE ENTIRE FORECAST PERIOD, MAKING INITIAL LANDFALL OVER HAINAN, 
CHINA, AROUND TAU 09, CROSS THE GULF OF TONKIN (GOT), THEN MAKE A 
FINAL LANDFALL OVER NORTHERN VIETNAM JUST AFTER TAU 36. THE 
FAVORABLE ENVIRONMENT WILL FUEL A MODEST INTENSIFICATION TO 45KTS IN 
THE GULF OF TONKIN BY TAU 24 AFTER A MOMENTARY WEAKENING TO 35KTS 
THROUGH HAINAN. AFTER THE FINAL LANDFALL OVER VIETNAM, THE SYSTEM 
WILL RAPIDLY ERODE MOSTLY DUE TO FRICTIONAL EFFECTS FROM THE RUGGED 
TERRAIN, LEADING TO DISSIPATION BY TAU 72. NUMERICAL MODELS REMAIN 
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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