WDPN31 PGTW 121500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 05W (KOGUMA) 
WARNING NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
    12JUN21. TROPICAL STORM (TS) 05W (KOGUMA), LOCATED APPROXIMATELY 
99 NM SOUTHEAST OF HANOI, VIETNAM, HAS TRACKED NORTHWESTWARD AT 17 KNOTS 
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) 
SHOWS THE SYSTEM HAS EXPANDED AND DEEPENED AS IT ACCELERATED ACROSS THE 
GULF OF TONKIN EVEN AS THE ASSOCIATED CONVECTION REMAINED SHEARED 
WESTWARD AND SOUTHWESTWARD OF THE LOW LEVEL CIRCULATION THAT HAS BECOME 
OBSCURED. THE INITIAL POSITION AND INTENSITY OF 35KTS ARE ASSESSED WITH 
FAIR CONFIDENCE BASED ON CONCENTRIC PGTW/RJTD DVORAK FIXES OF T2.5/35KTS, 
SUPPORTED BY COASTAL AND OFFSHORE SURFACE OBSERVATIONS, AND REFLECTS THE 
IMPROVED STATE. THE CYCLONE IS IN A MARGINAL ENVIRONMENT WITH GOOD 
WESTWARD AND EQUATORWARD OUTFLOW AND VERY WARM (30-31C) SST OFFSET BY 
MODERATE (15-20KTS) VERTICAL WIND SHEAR (VWS). TS 05W IS TRACKING ALONG 
THE SOUTHWESTERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) TO THE EAST-
NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE 
PREVIOUS PROGNOSTIC REASONING BULLETIN.
   B. TS KOGUMA WILL CONTINUE ON ITS CURRENT TRACK UNDER THE STR, MAKE 
LANDFALL OVER NORTHERN VIETNAM AROUND TAU 06, THEN TRACK INLAND INTO THE 
RUGGED VIETNAMESE TERRAIN. THE ROBUST OUTFLOW AND THE WARM SST IN THE 
GULF WILL OFFSET THE VWS AND MAY FUEL SLIGHT INTENSIFICATION PRIOR TO 
LANDFALL. AFTERWARD, LAND INTERACTION WITH THE RUGGED TERRAIN WILL RESULT 
IN A RAPID DECAY LEADING TO DISSIPATION BY TAU 24 AFTER IT PASSES JUST 
SOUTH OF HANOI. NUMERICAL MODELS ARE IN FAIR AGREEMENT WITH EVEN AND 
GRADUAL SPREADING TO 40NM AT TAU 12; AFTERWARD, ERRATIC DISPERSION AS 
THEY LOSE THE VORTEX OVER LAND. IN VIEW OF THIS, THERE IS FAIR CONFIDENCE 
IN THE JTWC TRACK FORECAST.//
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