WDPN32 PGTW 130900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 25W (VAMCO) WARNING NR 017// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 25W (VAMCO), LOCATED APPROXIMATELY 354 NM EAST OF DA NANG, VIETNAM, HAS TRACKED WESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TIGHTLY CURVED CONVECTIVE BANDS WRAPPING INTO A SMALL, CLOUD-OBSCURED EYE. THE MSI ALSO REVEALS A LARGE FIELD OF LOW-LEVEL STRATOCUMULUS EXTENDING ALL THE WAY TO HAINAN. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE WELL DEFINED EYE IN HIGH RESOLUTION VISIBLE IMAGERY. THE INITIAL INTENSITY REMAINS AT 75 KTS, AND IS ASSESSED WITH FAIR CONFIDENCE BASED ON ANALYSIS OF MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES BETWEEN T3.5 (55 KNOTS) AND T4.5 (77 KNOTS), WHILE THE ADVANCED DVORAK TECHNIQUE (ADT) ESTIMATES REMAIN UNREASONABLY HIGH AT T5.5. TY VAMCO IS TRACKING WESTWARD ALONG THE SOUTHERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST AND THROUGH A FAVORABLE ENVIRONMENT WITH LOW (5-10 KTS) VERTICAL WIND SHEAR, WARM (27-28 CELSIUS) SEA SURFACE TEMPERATURES (SST) AND WELL ESTABLISHED POLEWARD AND EQUATORWARD OUTFLOW ALOFT. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 25W WILL CONTINUE TO TRACK WEST TO WEST-NORTHWESTWARD THROUGH THE ENTIRETY OF THE FORECAST PERIOD UNDER THE STEERING INFLUENCE OF THE AFOREMENTIONED STR. THE ENVIRONMENT IS FORECAST TO REMAIN GENERALLY FAVORABLE DURING THE FIRST 12 HOURS OF THE FORECAST, WITH SSTS ABOVE 27 DEG CELSIUS, VWS IN THE LOW TO MODERATE RANGE AND MODERATE UPPER- LEVEL OUTFLOW, SUPPORTING A PERIOD OF STEADY INTENSITY. AFTER TAU 12 HOWEVER, ENVIRONMENT WILL SLOWLY DEGRADE, WITH THE PRIMARY FACTOR DRIVING THE WEAKENING TREND BEING THE STEADY DECREASE IN SSTS AS THE SYSTEM APPROACHES THE COAST OF VIETNAM. CURRENT SST MEASUREMENTS INDICATE SSTS DECREASE TO LESS THAN 26 DEG CELSIUS WEST OF THE 110 DEG LONGITUDE, AND AFTER THE SYSTEM CROSSES THIS POINT AFTER TAU 36, IT WILL QUICKLY WEAKEN THROUGH LANDFALL. THE SYSTEM IS FORECAST TO MAKE LANDFALL NEAR TAU 48 ALONG THE COAST OF NORTH-CENTRAL VIETNAM, HAVING WEAKENED TO 45 KNOTS. AS THE SYSTEM TRACKS OVER THE RUGGED TERRAIN OF NORTHERN VIETNAM AND INTO LAOS IT WILL BEGIN TO RAPIDLY DECAY DUE TO INCREASING VWS AND TERRAIN INTERACTION AND DISSIPATE BY TAU 72 OVER LAOS. NUMERICAL MODEL TRACK GUIDANCE IS IN FAIR AGREEMENT THROUGH TAU 36 BUT YIELDS POOR GUIDANCE THEREAFTER AS MODELS DIVERGE. SIGNIFICANTLY, THE UKMET ENSEMBLE TRACKS THE VORTEX NORTH TOWARDS THE CHINA/VIETNAM BORDER WHILE THE REMAINDER OF THE MODEL TRACKERS ARE WELL CLUSTERED NEAR THE JTWC FORECAST TRACK. HOWEVER, DUE TO THE PREPONDERANCE OF THE MODEL GUIDANCE BEING TIGHTLY PACKED, THERE IS OVERALL HIGH CONFIDENCE IN THE JTWC FORECAST TRACK WHICH LIES ON THE NORTHERN EDGE OF THE GUIDANCE ENVELOPE.// NNNN