WDPN32 PGTW 241500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING NR 003// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (TWENTYONE), LOCATED APPROXIMATELY 467 NM EAST OF MANILA, PHILIPPINES, HAS TRACKED WESTWARD AT 10 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS WELL-DEFINED BANDING WRAPPING INTO A CONSOLIDATING, PARTIALLY-EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). A 241157Z OSCAT IMAGE REVEALS 30-35 KT WINDS IN THE NORTHEAST QUADRANT AND WEAKER WINDS ELSEWHERE. A 240952Z SSMIS 37 GHZ MICROWAVE IMAGE SHOWS A SMALL NOTCH FEATURE. THE INITIAL POSITION IS BASED ON THE PARTIALLY-EXPOSED LLCC AND IS IN GOOD AGREEMENT WITH THE PGTW AND RJTD FIXES. THE INITIAL INTENSITY IS ASSESSED AT 35 KTS BASED ON THE AFOREMENTIONED OSCAT, A KNES DVORAK CURRENT INTENSITY ESTIMATE OF T2.5 (35 KTS), AND A 241140Z ADVANCED DVORAK TECHNIQUE ESTIMATE OF T2.2 (32 KTS). UPPER LEVEL ANALYSIS IS CHARACTERIZED BY WESTWARD OUTFLOW AND MODERATE (15-20 KTS) VERTICAL WIND SHEAR (VWS). COMBINED WITH WARM (29-30 CELSIUS) SEA SURFACE TEMPERATURES (SST), THE ENVIRONMENT IS MODERATELY FAVORABLE FOR CONTINUED DEVELOPMENT. TS 21W IS TRACKING GENERALLY WESTWARD UNDER A SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST WITH AN EXTENSION TO THE NORTH. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 21W IS EXPECTED TO CONTINUE CONSOLIDATING AS IT TRACKS GENERALLY WESTWARD ALONG THE PERIPHERY OF THE STR. AS FAVORABLE CONDITIONS PERSIST, THE SYSTEM SHOULD INTENSIFY TO 60 KTS PRIOR TO MAKING LANDFALL OVER THE EASTERN PHILIPPINES NEAR TAU 24. AS IT TRANSITS OVER THE ARCHIPELAGO, THE SYSTEM SHOULD MAINTAIN INTENSITY (OR POSSIBLY WEAKEN SLIGHTLY) BEFORE THE SYSTEM RE-EMERGES OVER THE SOUTH CHINA SEA WHERE IT IS EXPECTED TO RE-INTENSIFY TO A PEAK OF 80 KNOTS BY TAU 72. NUMERICAL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 24. AFTERWARDS, THE MODEL SOLUTIONS BEGIN TO STEADILY DIVERGE AS SOME MODELS SHOW A WEAKENING STEERING STR, RESULTING IN A SPREAD OF 103 NM BY TAU 72. BASED ON THIS INCREASING SPREAD, THERE IS FAIR CONFIDENCE IN THE NEAR TERM FORECAST. C. AFTER TAU 72, NUMERICAL GUIDANCE CONTINUES TO DIVERGE AS THE SYSTEM APPROACHES CENTRAL VIETNAM. THE JTWC FORECAST TRACK LIES CLOSE TO THE MULTI-MODEL CONSENSUS TRACK, TRANSITING TS 21W WESTWARD THEN NORTH-WESTWARD ALONG THE STEERING STR PERIPHERY. PERSISTENT FAVORABLE ENVIRONMENTAL CONDITIONS WILL ALLOW THE SYSTEM TO ONLY SLIGHTLY WEAKEN BY TAU 96 DUE TO INTERACTION WITH THE COAST, MAKING LANDFALL JUST AFTER TAU 96. AFTERWARDS, THE RUGGED TERRAIN OF VIETNAM WILL STEADILY WEAKEN THE SYSTEM AS IT TRACKS INLAND. DUE TO THE MODEL SPREAD IN THE LATER TAUS, THERE IS OVERALL LOW CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN