Tropical Storm NANGKA Advisory wto., 13.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA)
WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (NANGKA), LOCATED APPROXIMATELY 235 NM
SOUTH-SOUTHWEST OF HONG KONG, HAS TRACKED WEST-NORTHWESTWARD AT 12
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY SHOWS THE SYSTEM HAS MAINTAINED ITS OVERALL CONVECTIVE
SIGNATURE WITH LONG FRAGMENTED RAIN BANDS FEEDING IN MOSTLY FROM THE
SOUTH INTO A LOW LEVEL CIRCULATION (LLC) THAT IS OBSCURED BY THE
CENTRAL CONVECTION. THE INITIAL POSITION IS PLACED WITH FAIR
CONFIDENCE BASED ON THE ABOVE ANIMATION AND ON A LLC FEATURE IN THE
122327Z SSMIS MICROWAVE PASS. THE INITIAL INTENSITY OF 40KTS IS HELD
A BIT HIGHER THAN THE PGTW AND RJTD DVORAK INTENSITY ESTIMATES OF
T2.5/35KTS TO REFLECT THE SUSTAINED CONVECTIVE STRUCTURE. ANALYSIS
INDICATES A FAVORABLE ENVIRONMENT WITH LOW (10-15KT) VERTICAL WIND
SHEAR, GOOD EQUATORWARD OUTFLOW, AND WARM (29-30C) SST IN THE SOUTH
CHINA SEA. 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. THE STR WILL REMAIN THE PRIMARY STEERING MECHANISM FOR TS
NANGKA AS IT DRIVES IT ON ITS CURRENT WEST-NORTHWESTWARD TRACK FOR
THE ENTIRE FORECAST PERIOD, MAKING INITIAL LANDFALL OVER HAINAN,
CHINA, AROUND TAU 09, CROSS THE GULF OF TONKIN (GOT), THEN MAKE A
FINAL LANDFALL OVER NORTHERN VIETNAM JUST AFTER TAU 36. THE
FAVORABLE ENVIRONMENT WILL FUEL A MODEST INTENSIFICATION TO 45KTS IN
THE GULF OF TONKIN BY TAU 24 AFTER A MOMENTARY WEAKENING TO 35KTS
THROUGH HAINAN. AFTER THE FINAL LANDFALL OVER VIETNAM, THE SYSTEM
WILL RAPIDLY ERODE MOSTLY DUE TO FRICTIONAL EFFECTS FROM THE RUGGED
TERRAIN, LEADING TO DISSIPATION BY TAU 72. NUMERICAL MODELS REMAIN
IN TIGHT AGREEMENT WITH MINIMAL SPREADING AFTER TAU 36. THE JTWC
FORECAST TRACK LIES NEAR THE MULTI-MODEL CONSENSUS WITH OVERALL HIGH
CONFIDENCE.//
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