Tropical Storm NANGKA Advisory Wed Oct 14

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA)
WARNING NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (NANGKA), LOCATED APPROXIMATELY 264 NM
SOUTHWEST OF HONG KONG, HAS TRACKED WESTWARD AT 07 KNOTS OVER THE
PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE DEPICTS A BROAD
SYSTEM WITH WELL DEFINED BANDING SPIRALING INTO A WELL DEFINED
CENTRAL LLCC. HOWEVER THE CONVECTIVE SIGNATURE IS SOMEWHAT
FRAGMENTED, WITH NO CENTRAL DEEP CONVECTION AND ONLY FRAGMENTED
AREAS OF CONVECTION WELL DISPLACED FROM THE LLCC. THE INITIAL
POSITION IS PLACED WITH MODERATE CONFIDENCE BASED ON ANALYSIS OF
THE CONVECTIVE BANDING SEEN IN THE ANIMATED MSI, WITH SUPPORT
FROM A LOW-RESOLUTION 120552Z ATMS 165 GHZ MICROWAVE IMAGE, WHICH
INDICATED A BROAD AREA OF CURVED CONVECTIVE BANDING ON THE
SOUTHERN SIDE OF THE CENTER. THE INITIAL INTENSITY IS ASSESSED
AT 45 KNOTS WITH HIGH CONFIDENCE BASED ON A SLIGHT HEDGE UPWARD
FROM THE PGTW T2.5 DVORAK INTENSITY ESTIMATE IN LIGHT OF AN
ADVANCED DVORAK TECHNIQUE ESTIMATE OF T3.0 AND A SATELLITE
CONSENSUS ESTIMATE OF 45 KNOTS. ENVIRONMENTAL ANALYSIS INDICATES
THE SYSTEM LIES IN A GENERALLY FAVORABLE ENVIRONMENT WITH LOW
(10-15KT) VERTICAL WIND SHEAR, GOOD DIVERGENT WESTWARD OUTFLOW,
AND WARM (29-30C) SST IN THE SOUTH CHINA SEA JUST EAST OF HAINAN
ISLAND. 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. IN THE NEAR-TERM, TS 18W IS FORECAST TO MAKE LANDFALL ALONG
THE EASTERN SHORE OF HAINAN ISLAND WITHIN THE NEXT HOURS AS IT
CONTINUES TRACKING GENERALLY WESTWARD ALONG THE SOUTHERN
PERIPHERY OF THE DEEP-LAYER STR TO THE NORTH. AFTER LANDFALL,
SOME SLIGHT WEAKENING IS EXPECTED AS THE LLCC INTERACTS WITH THE
RUGGED TERRAIN OF HAINAN, DISRUPTING THE PRIMARY CIRCULATION
CENTER AND CUTTING OFF THE ENERGY SUPPLY. BY ROUGHLY TAU 18
HOWEVER, THE CIRCULATION WILL REEMERGE INTO THE SOUTHERN GULF OF
TONKIN AND REINTENSIFY UNDER FAVORABLE ENVIRONMENTAL CONDITIONS
OF VERY WARM (29-30 DEG CELSIUS), LOW VWS AND DIVERGENT WESTWARD
OUTFLOW. THE SYSTEM IS FORECAST TO REACH A PEAK OF 50 KNOTS BY
TAU 36 BEFORE MAKING LANDFALL ALONG THE NORTH VIETNAMESE
COASTLINE PRIOR TO TAU 48. THERE IS A POSSIBILITY THAT THE
INTENSITY COULD REACH A SLIGHTLY HIGHER INTENSITY NEAR 55 KNOTS
JUST BEFORE LANDFALL, WHICH IS NOT CAPTURED BY THE FORECAST TAUS.
AFTER MAKING LANDFALL OVER NORTHERN VIETNAM, THE SYSTEM WILL
RAPIDLY WEAKEN OVER THE MOUNTAINOUS TERRAIN OF VIETNAM AND
DISSIPATING AS A TROPICAL CYCLONE OVER NORTHERN LAOS BY TAU 72.
NUMERICAL MODELS REMAIN IN GOOD AGREEMENT THOUGH THE ENTIRE
FORECAST PERIOD WITH ONLY MINIMAL SPREAD AFTER LANDFALL IN
VIETNAM. THE JTWC FORECAST TRACK LIES JUST SOUTH OF THE
MULTI-MODEL CONSENSUS THROUGH LANDFALL AND THEN JUST NORTH
THEREAFTER, WITH HIGH CONFIDENCE.//
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