WDPN33 PGTW 130900
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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