WDPN33 PGTW 120900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA) 
WARNING NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (NANGKA), LOCATED APPROXIMATELY 277 NM 
SOUTH-SOUTHEAST OF HONG KONG, HAS TRACKED WESTWARD AT 13 KNOTS OVER 
THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS 
A CONSOLIDATING SYSTEM WITH IMPROVED CONVECTIVE BANDING WRAPPING 
INTO THE LOW-LEVEL CIRCULATION CENTER (LLCC). A 120826Z SSMIS 91GHZ 
COLOR COMPOSITE IMAGE INDICATES FRAGMENTED CONVECTIVE BANDING 
WRAPPING INTO THE LLCC. THE INITIAL INTENSITY IS ASSESSED AT 35 
KNOTS, HEDGED ABOVE THE PGTW/RJTD DVORAK INTENSITY ESTIMATES OF T2.0 
(30 KNOTS), BASED ON A 120519Z SATCON ESTIMATE OF 38 KNOTS AND A 
120600Z ADT ESTIMATE OF 2.3/2.5 (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 60 KTS BY TAU 24. BY TAU 36, THE SYSTEM WILL WEAKEN AS 
IT TRACKS GRADUALLY 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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