WDPN31 PGTW 111500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (CONSON) WARNING NR 023// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 15.4N 109.1E INITIAL INTENSITY: 40 KTS GEOGRAPHIC REFERENCE: 59 NM SOUTHEAST OF DA NANG, VIETNAM MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 04 KTS SIGNIFICANT WAVE HEIGHT: 18 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY AND ANIMATED VIETNAMESE RADAR DATA INDICATE THE CENTER OF TS 18W IS JUST OFFSHORE NEAR CU LAO RE ISLAND. THE LOW LEVEL CIRCULATION CENTER (LLCC) REMAINS FULLY EXPOSED, WITH MODERATE CONVECTIVE ACTIVITY SHEARED TO THE WEST BY PERSISTENT MODERATE EASTERLY SHEAR. THE EXPOSED LLCC COMBINED WITH THE RADAR DATA LENT HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY OF 40 KNOTS IS ASSESSED WITH MEDIUM CONFIDENCE, BASED ON A BLEND OF MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES OF T3.0 (45 KNOTS) AND AN ADT ESTIMATE OF 34 KNOTS. THE SYSTEM LIES IN AN OVERALL MARGINAL ENVIRONMENT, WITH PERSISTENT MODERATE VWS OFFSETTING STRONG WESTWARD OUTFLOW AND WARM SSTS. OHC VALUES ARE LOW CLOSE TO THE COAST, CONTRIBUTING TO THE WEAKENING TREND AS THE SYSTEM HAS SLOWED AS IT APPROACHES THE COASTLINE. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: LOW TO MID-LEVEL SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTH. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS RJTD: T3.0 - 45 KTS CIMSS ADT: 34 KTS AT 111140Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 20-25 KTS SST: 27-28 CELSIUS OUTFLOW: STRONG WESTWARD OTHER FACTORS: LOW OHC WATERS AND TERRAIN INTERACTION. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TS CONSON IS FORECAST TO CONTINUE TRACKING WEST-SOUTHWEST AND MAKE LANDFALL NEAR BINH SON, VIETNAM WITHIN THE NEXT 6 TO 12 HOURS. OVER THE NEXT 24 HOURS THE STEERING RIDGE TO THE NORTH IS EXPECTED TO WEAKEN, LESSENING THE STEERING GRADIENT, AND AS THE SYSTEM IS RELATIVELY SHALLOW, THE TRACK AFTER LANDFALL WILL BE HIGHLY INFLUENCED BY THE LOCAL TERRAIN. THUS, ONCE OVER LAND, THE TRACK IS EXPECTED TO TURN NORTHWESTWARD, PARALLELING THE 2600 METER MOUNTAINS THAT LIE TO THE WEST OF BINH SON. TS 18W IS STRUGGLING TO MAINTAIN INTENSITY IN THE FACE OF PERSISTENT MODERATE EASTERLY SHEAR, AND ONCE OVER LAND THIS PROCESS WILL ACCELERATE, LEADING TO DISSIPATION OVER THE RUGGED TERRAIN BY TAU 48. AS WAS THE CASE WITH THE PREVIOUS FORECAST, THERE REMAINS A POSSIBLE ALTERNATE SCENARIO, IN WHICH THE STEERING RIDGE BREAKS DOWN RELATIVELY QUICKLY, LEADING TO A WEAK STEERING PATTERN, ALLOWING THE SYSTEM TO MAKE A BRIEF LANDFALL BEFORE RETURNING TO SEA, REMAINING QUASI-STATIONARY FOR A PERIOD AND THEN TRACKING NORTHWESTWARD PARALLEL TO THE COASTLINE. MODEL DISCUSSION: TRACK GUIDANCE CONTINUES TO SHOW A HIGH AMOUNT OF UNCERTAINTY, WITH A DRAMATIC AND INCREASING SPREAD IN THE DETERMINISTIC GUIDANCE. NAVGEM CONTINUES TO DEPICT A SOUTHWESTWARD TRACK INTO NORTHERN CAMBODIA. GFS, HWRF, ECMWF AND THE GFS ENSEMBLE NOW SHOW THE SYSTEM STRADDLING THE COAST THROUGH TAU 24, THEN TURNING SHARPLY NORTH AND PARALLELING THE COAST THROUGH TAU 48 BEFORE A LANDFALL NORTH OF HUE. THE ECMWF ENSEMBLE, UKMET AND ENSEMBLE AND GALWEM ALL SHOW A NORTHWESTWARD TRACK OVER LAND. THE JTWC FORECAST CLOSELY FOLLOWS THE UKMET AND ECMWF ENSEMBLE MEAN TRACKERS WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS IN GOOD AGREEMENT ON A WEAKENING TREND THROUGH THE FORECAST PERIOD, WITH THE EXCEPTION OF HWRF WHICH DEPICTS A 5-KNOT INTENSIFICATION PRIOR TO LANDFALL. THE JTWC FORECAST LIES CLOSE TO THE CONSENSUS MEAN WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM// NNNN