WDPN33 PGTW 151500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (BEBINCA) WARNING 
NR 13//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 20W (BEBINCA), LOCATED APPROXIMATELY 200 NM WEST-
SOUTHWEST OF HONG KONG, HAS TRACKED WESTWARD AT 06 KNOTS OVER THE 
PAST SIX HOURS. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE 
BASED ON ANIMATED ENHANCED INFRARED SATELLITE IMAGERY WHICH DEPICTS 
PERSISTENT CONVECTION BEING SHEARED TO THE SOUTHWEST OF THE LOW 
LEVEL CIRCULATION CENTER (LLCC). THE INITIAL INTENSITY OF 35 KNOTS 
IS CONSISTENT WITH PGTW AND RJTD DVORAK CURRENT INTENSITY ESTIMATES 
OF T2.5 (35 KNOTS). ENVIRONMENTAL CONDITIONS ARE MARGINAL WITH GOOD 
EQUATORWARD OUTFLOW AND MODERATE (20-25 KT) VERTICAL WIND SHEAR 
(VWS). SEA SURFACE TEMPERATURE (28-29 CELSIUS) SUPPORTS 
INTENSIFICATION. TS 20W IS TRACKING WESTWARD ALONG THE SOUTHERN 
PERIPHERY OF A SUBTROPICAL RIDGE (STR) TO THE NORTH.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY FROM THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE. 
   B. UNTIL DISSIPATING OVER LAOS BY TAU 72, TS 20W IS FORECAST TO 
CONTINUE TRACKING WESTWARD ALONG THE SOUTHERN PERIPHERY OF THE STR TO 
THE NORTH. BETWEEN TAU 24 AND 36, THERE IS A CHANCE FOR SOME 
INTENSIFICATION AS VWS IS EXPECTED TO DECREASE TO 10-15 KTS. PRIOR 
TO TAU 48, THE SYSTEM WILL MAKE LANDFALL OVER VIETNAM AND BEGIN TO 
WEAKEN UNTIL TAU 72 WHEN IT FALLS TO 20 KNOTS. AMONG THE MODEL 
GUIDANCE, THERE ARE TWO OUTLIERS (HWRF AND JGSM) THAT CURL THE 
CYCLONE TO THE NORTH. THE REMAINING MODELS GENERALLY AGREE THAT TS 
20W WILL TRACK WESTWARD, SIMILAR TO THE JTWC FORECAST TRACK, 
HOWEVER, PRIMARILY DUE TO VARIATIONS IN ALONG TRACK SPEED, THE 
SPREAD EXCEEDS 500 NM BY TAU 72. BASED ON THE LARGE SPREAD OF THE 
MODEL GUIDANCE, CONFIDENCE IN THE JTWC FORECAST TRACK IS LOW.//
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