WDPN33 PGTW 132100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA) 
WARNING NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (NANGKA), LOCATED APPROXIMATELY 183 NM 
EAST-SOUTHEAST OF HANOI, VIETNAM, HAS TRACKED WEST-NORTHWESTWARD AT 
17 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) 
SATELLITE IMAGERY SHOWS THE SYSTEM HAS ACCELERATED THROUGH THE 
ISLAND OF HAINAN AND EXITED INTO THE GULF OF TONKIN WITH MINOR 
DEGRADATION. CONVECTION WRAPPING INTO A DEFINED LOW LEVEL 
CIRCULATION (LLC) HAS QUICKLY RESUMED ALTHOUGH THE MAIN FEEDER BAND 
HAS SEVERED AND REMAINED IN THE SOUTH CHINA SEA. THE INITIAL 
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE LLC FEATURES IN 
THE EIR LOOP AND A COMPOSITE RADAR LOOP FROM CNA. THE INITIAL 
INTENSITY OF 45KTS IS BASED ON THE HIGH END OF AGENCY DVORAK 
ESTIMATES OF T2.5/35KTS TO T3.0/45KTS. ENVIRONMENTAL ANALYSIS 
INDICATES THE SYSTEM IS IN A GENERALLY FAVORABLE ENVIRONMENT WITH 
LOW (10-15KT) VERTICAL WIND SHEAR, DIVERGENT WESTWARD OUTFLOW, AND 
WARM (30C) SST. 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. TS NANGKA WILL TRACK WESTWARD UNDER THE STR THROUGH THE GULF 
OF TONKIN AND MAKE A FINAL LANDFALL OVER NORTHERN VIETNAM NEAR TAU 
18. THE FAVORABLE ENVIRONMENT WILL FUEL A SECONDARY PEAK TO 50KTS BY 
TAU 12; AFTERWARD, LAND INTERACTION WILL RAPIDLY ERODE THE SYSTEM TO 
DISSIPATION BY TAU 36 AFTER IT DRAGS ACROSS THE RUGGED TERRAIN OF 
VIETNAM AND LAOS. NUMERICAL MODELS REMAIN IN TIGHT AGREEMENT, 
LENDING HIGH CONFIDENCE TO THE JTWC TRACK FORECAST.//
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