WDPN31 PGTW 280300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING NR 08// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (TWENTYONE), LOCATED APPROXIMATELY 310 NM SOUTHWEST OF MANILA, PHILIPPINES, HAS TRACKED WEST-NORTHWESTWARD AT 20 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THE SYSTEM HAS STARTED TO CONSOLIDATE AS THE SYSTEM TRACKED FURTHER OUT TO SEA WHILE CONVECTIVE BANDING HAS INCREASED ALONG THE NORTHERN PERIPHERY. A RECENT 280009Z SSMIS MICROWAVE IMAGE SHOWS THE LOW LEVEL CIRCULATION CENTER (LLCC) REMAINS POORLY ORGANIZED DESPITE AN OVERALL INCREASE IN CONVECTIVE STRUCTURE. LOW CONFIDENCE REMAINS IN THE CURRENT POSITION DUE TO THE POOR DEFINITION OF THE LLCC AS SEEN IN THE MSI LOOP AND THE MICROWAVE PASS. THE INITIAL INTENSITY HAS BEEN INCREASED TO 35 KNOTS BASED ON DVORAK INTENSITY ESTIMATES FROM PGTW AND KNES AND THE INCREASE IN STRUCTURE. UPPER-LEVEL ANALYSIS INDICATES TS 21W REMAINS IN A FAVORABLE ENVIRONMENT WITH LOW TO MODERATE (10 TO 20 KNOTS) VERTICAL WIND SHEAR WHICH IS BEING OFFSET BY EXCELLENT POLEWARD OUTFLOW. TS 21W IS CURRENTLY TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) ANCHORED TO THE NORTH. 3. FORECAST REASONING. A. NO CHANGE TO FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 21W WILL CONTINUE TRACKING WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE OF THE STR THROUGH THE REMAINDER OF THE FORECAST. THE FAVORABLE UPPER LEVEL CONDITIONS ARE EXPECTED TO PERSIST WHICH WILL ALLOW THE SYSTEM TO GRADUALLY INTENSIFY AS IT TRACKS INTO THE MORE OPEN WATERS OF THE SOUTH CHINA SEA. BY TAU 48, THE SYSTEM IS EXPECTED TO MAKE LANDFALL ACROSS THE COAST OF VIETNAM AND WILL RAPIDLY DISSIPATE AS IT TRACKS INLAND. AVAILABLE MODEL GUIDANCE REMAINS IN GOOD AGREEMENT, LENDING TO HIGH CONFIDENCE IN THE JTWC FORECAST TRACK WHICH IS POSITIONED CLOSE TO THE MULTI-MODEL CONSENSUS.// NNNN