WDPN33 PGTW 121500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (NANGKA), LOCATED APPROXIMATELY 246 NM 
SOUTH OF HONG KONG, HAS TRACKED WESTWARD AT 15 KNOTS OVER THE PAST 
SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS A 
CONSOLIDATING SYSTEM WITH IMPROVED CONVECTIVE BANDING WRAPPING 
INTO THE LOW-LEVEL CIRCULATION CENTER (LLCC). A 121053Z SSMIS 91GHZ 
IMAGE INDICATES FRAGMENTED CONVECTIVE BANDING WRAPPING INTO THE 
LLCC. THE INITIAL INTENSITY IS ASSESSED AT 35 KNOTS BASED ON A PGTW 
DVORAK INTENSITY ESTIMATE OF T2.5 (35 KNOTS). UPPER-LEVEL ANALYSIS 
REVEALS A FAVORABLE ENVIRONMENT WITH LOW VERTICAL WIND SHEAR, GOOD 
EQUATORWARD OUTFLOW AND EXPANDING POLEWARD OUTFLOW. ADDITIONALLY, 
SST (29-30C) VALUES ARE CONDUCIVE FOR FURTHER DEVELOPMENT. TS 18W IS 
TRACKING ALONG THE SOUTHERN PERIPHERY OF A DEEP-LAYERED SUBTROPICAL 
RIDGE (STR) POSITIONED TO THE NORTH.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. THE AFOREMENTIONED STR WILL ACT AS THE STEERING MECHANISM 
THROUGHOUT THE ENTIRE FORECAST PERIOD. TS 18W IS FORECAST TO TRACK 
GENERALLY WESTWARD THROUGH THE TAU 72 AS IT INTENSIFIES TO A PEAK 
INTENSITY OF 55 KTS BY TAU 24. AFTER TAU 24, THE SYSTEM WILL WEAKEN 
AS IT TRACKS OVER HAINAN ISLAND AND INTO THE GULF OF TONKIN. 
TS 18W WILL MAKE LANDFALL OVER NORTHEAST VIETNAM NEAR TAU 60 AND 
WILL WEAKEN AS IT TRACKS INLAND. NUMERICAL MODEL GUIDANCE REMAINS IN 
TIGHT AGREEMENT THROUGH TAU 72. 
   C. IN THE EXTENDED RANGE, TS 18W WILL TRACK FURTHER INLAND OVER 
MOUNTAINOUS TERRAIN AND WILL DISSIPATE BY TAU 96. THE JTWC FORECAST 
TRACK LIES NEAR THE MULTI-MODEL CONSENSUS WITH OVERALL HIGH 
CONFIDENCE.//
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