WDPN32 PGTW 240900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 002// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 21W (TWENTYONE), LOCATED APPROXIMATELY 443 NM NORTHWEST OF KOROR, HAS TRACKED WEST-NORTHWESTWARD AT 14 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS WELL DEFINED BANDING WRAPPING INTO A CONSOLIDATING LOW LEVEL CIRCULATION CENTER. A 240524Z AMSR2 89 GHZ PARTIAL MICROWAVE IMAGE SHOWS DEFINED BANDING WRAPPING INTO THE LLCC FROM THE NORTHWESTERN QUADRANT, LENDING GOOD CONFIDENCE TO THE INITIAL POSITION THAT IS IN GOOD AGREEMENT WITH THE PGTW AND RJTD FIXES. THE INITIAL INTENSITY REMAINS ASSESSED AT 30 KTS BASED ON MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES OF T1.5 (25 KTS) BY PGTW/RJTD AND A 240540Z ADVANCED DVORAK TECHNIQUE ESTIMATE OF T2.0 (30 KTS). UPPER LEVEL ANALYSIS REVEALS IMPROVING 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. TD 21W IS TRACKING GENERALLY NORTHWESTWARD 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. TD 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 36. AS IT TRANSITS OVER THE ARCHIPELAGO, A SLIGHT WEAKENING IS EXPECTED BEFORE THE SYSTEM RE-EMERGES OVER THE SOUTH CHINA SEA WHERE RE- INTENSIFICATION IS FORECAST TO REACH A PEAK OF 75 KNOTS AT TAU 72. NUMERICAL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 36; HOWEVER, THE MODEL SOLUTIONS BEGIN TO DIVERGE AFTERWARDS AS SOME MODELS SHOW A WEAKENING STEERING STR. DUE TO THE INCREASING SPREAD IN THE GUIDANCE, THERE IS FAIR CONFIDENCE IN THE NEAR-TERM FORECAST. C. AFTER TAU 72, NUMERICAL GUIDANCE CONTINUES TO DIVERGE AS SOME SOLUTIONS MORE AGGRESSIVELY WEAKEN THE STR. THE JTWC FORECAST LIES CLOSE TO THE MULTI-MODEL CONSENSUS TRACK AND TRANSITS TD 21W WESTWARD THEN NORTH-WESTWARD ALONG THE STR PERIPHERY. THROUGHOUT THIS TIME PERIOD, PERSISTENT FAVORABLE ENVIRONMENTAL CONDITIONS WILL ALLOW THE SYSTEM TO MAINTAIN ITS INTENSITY UNTIL LANDFALL OVER CENTRAL VIETNAM PRIOR TO TAU 120. AFTERWARDS, INTERACTION WITH 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