WDPN32 PGTW 181500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 24W (RAGASA) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 15.9N 133.0E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 849 NM SOUTHEAST OF TAIPEI, TAIWAN MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 04 KTS SIGNIFICANT WAVE HEIGHT: 13 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL DEPRESSION 24W (RAGASA) WITH WIDESPREAD DEEP CONVECTION ALONG THE SOUTHERN PERIPHERY OF THE SYSTEM. AN 181132Z METOP-B ASCAT IMAGE REVEALED A LARGE SWATH OF 25 KNOT WINDS WITHIN THE EASTERN SEMICIRCLE WITH A PATCH OF 30 KTS CONFINED TO THE SOUTHEASTERN QUADRANT. ENVIRONMENTAL ANALYSIS INDICATES THAT 24W IS IN A FAVORABLE ENVIRONMENT CHARACTERIZED BY STRONG EQUATORWARD OUTFLOW ALOFT, LOW (10-15 KTS) VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA SURFACE TEMPERATURES. THERE CURRENTLY IS DRY AIR OVER THE NORTHERN PORTION OF THE VORTEX, WHICH IS HINDERING CONVECTIVE DEVELOPMENT IN THAT REGION. THE INITIAL POSITION AND INTENSITY ARE BOTH ASSESSED WITH HIGH CONFIDENCE BASED ON THE MENTIONED METOP-B ASCAT IMAGE. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: BUILDING SUBTROPICAL RIDGE (STR) TO THE NORTH AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T2.0 - 30 KTS KNES: T1.5 - 25 KTS CIMSS D-PRINT: 28 KTS AT 181200Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 10-15 KTS SST: 29-30 CELSIUS OUTFLOW: STRONG EQUATORWARD OTHER FACTORS: DRY AIR TO THE NORTH ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: 24W IS FORECAST TO TRACK GENERALLY WEST-NORTHWESTWARD OVER THE NEXT 72 HOURS. TRACK SPEEDS ARE EXPECTED TO BE RELATIVELY SLOW DUE TO THE WEAK STEERING PATTERN AS THE SUBTROPICAL RIDGE TO THE NORTH GRADUALLY BUILDS. AFTER TAU 72, THE RIDGE CENTERED SOUTH OF KYUSHU WILL HAVE MORE INFLUENCE ON THE MOTION OF THE VORTEX, CAUSING TRACK SPEEDS TO PICK UP AND THE TRACK TO TURN MORE WESTWARD. A WESTWARD TRACK THROUGH THE LUZON STRAIT IS EXPECTED TO PERSIST FROM TAU 96 THROUGH THE REMAINDER OF THE FORECAST PERIOD. REGARDING INTENSITY, 24W IS FORECAST TO STEADILY INTENSIFY THROUGH TAU 36 TO AROUND 60 DUE TO THE FAVORABLE ENVIRONMENT. AFTER TAU 36, POLEWARD OUTFLOW IS EXPECTED TO MARKEDLY INCREASE, ALLOWING FOR RAPID INTENSIFICATION THROUGH TAU 72 TO AROUND 105 KTS. SEA SURFACE TEMPERATURES WILL REMAIN VERY WARM THROUGH THE ENTIRE FORECAST, ALLOWING FOR PRIME CONDITIONS FOR INTENSIFICATION. AS THE SYSTEM TRACKS THROUGH THE LUZON STRAIT, 24W IS FORECAST TO REACH A PEAK INTENSITY OF NEAR 115 KTS. NEAR THE END OF THE FORECAST, NORTHEASTERLY SHEAR IS EXPECTED TO SLIGHTLY INCREASE TO AROUND 15-20 KTS, CAUSING THE INTENSIFICATION TREND TO HALT. MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGHOUT THE FORECAST WITH A 128 NM CROSS-TRACK SPREAD AT TAU 72 AND A 182 NM CROSS-TRACK SPREAD AT TAU 120. THERE ARE ALSO NO SIGNIFICANT OUTLIERS, ALLOWING FOR THE JTWC TRACK FORECAST TO BE PLACED WITH OVERALL MEDIUM CONFIDENCE. INTENSITY GUIDANCE, ON THE OTHER HAND, HAS QUITE A BIT MORE UNCERTAINTY. THOUGH ALL MODELS DEPICT INTENSIFICATION THROUGH TAU 96, THE RATE OF INTENSIFICATION VARIES. MANY RAPID INTENSIFICATION AIDS ARE TRIGGERING, SUGGESTING AS HIGH AS 100 KTS IN 48 HOURS. HAFS-A HAS A PEAK INTENSITY OF 140 KTS AT TAU 84 AND IS THE MOST AGGRESSIVE MODEL IN TERMS OF THE PEAK. THE JTWC INTENSITY FORECAST IS PLACED IN LINE WITH THE MULTI-MODEL CONSENSUS THROUGH TAU 48 AND THEN ABOUT 10-15 KTS HIGHER AFTERWARD WITH OVERALL LOW CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 00-72 HR: LOW INTENSITY 72-120 HR: LOW// NNNN