WDPN32 PGTW 230900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (DIANMU) WARNING NR 004// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 14.3N 110.2E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 150 NM SOUTHEAST OF DA NANG, VIETNAM MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 13 KTS SIGNIFICANT WAVE HEIGHT: 12 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS A PARTIALLY EXPOSED LOW LEVEL CIRCULATION SYSTEM WITH DEEP CONVECTION OFFSET NORTHWESTWARD OF THE LOW LEVEL CIRCULATION CENTER. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON A 230510Z GMI 89GHZ MICROWAVE IMAGE AND RADAR COVERAGE. THE INITIAL INTENSITY IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE OVERALL ASSESSMENT OF AGENCY AND AUTOMATED DVORAK ESTIMATES. THE SYSTEM IS IN A MARGINAL ENVIRONMENT BEFORE LANDFALL WITH WARM SSTS OFFSET BY WEAK OUTFLOW AND MEDIUM VERTICAL WIND SHEAR (15-20 KNOTS). THE CYCLONE IS TRACKING ALONG THE SOUTHWEST PERIPHERY OF A SUBTROPICAL RIDGE LOCATED TO THE NORTHEAST. INITIAL WIND RADII BASIS: 230130Z ASCAT METOP-A CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE NORTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.5 - 25 KTS CIMSS SATCON: 39 KTS AT 230531Z CIMSS ADT: 34 KTS AT 230550Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 15-20 KTS SST: 28-29 CELSIUS OUTFLOW: WEAK POLEWARD 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: TD 21W IS EXPECTED TO CONTINUE ON ITS CURRENT TRACK AND MAKE LANDFALL SOUTH OF DANANG BEFORE TAU 12 AND TRANSITION ACROSS THE RUGGED TERRAIN OF VIETNAM AND INTO CAMBODIA. THE MARGINAL ENVIRONMENT WILL LIKELY MAINTAIN INTENSITY WHILE OVER WATER BEFORE BEGINNING DISSIPATION DUE TO LAND INTERACTION. LAND INTERACTION WILL ERODE THE SYSTEM TO DISSIPATION BY TAU 36. MODEL DISCUSSION: NUMERICAL TRACK MODELS ARE IN HIGH AGREEMENT THROUGH THE REMAINDER OF THE FORECAST. INTENSITY GUIDANCE IS IN MEDIUM AGREEMENT DUE TO COMPETING ENVIRONMENTAL PARAMETERS. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH INTENSITY 0 - 72 HR: MEDIUM// NNNN