WDPN33 PGTW 291500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (BANG-LANG) WARNING NR 014// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 25.3N 168.6E INITIAL INTENSITY: 75 KTS GEOGRAPHIC REFERENCE: 377 NM NORTH-NORTHEAST OF WAKE ISLAND MOVEMENT PAST 6 HOURS: NORTHWARD AT 13 KTS SIGNIFICANT WAVE HEIGHT: 26 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: THE CLOUD PATTERN OF TYPHOON (TY) 20W HAS BECOME LESS ORGANIZED. THE NASCENT EYE FEATURE THAT WAS PREVIOUSLY VISIBLE IS NO LONGER APPARENT. WHILE VERY DEEP CONVECTION CONTINUES TO FLARE ALONG THE NORTHERN AND EASTERN PERIPHERIES, WITH OVERSHOOTING TOPS AS COLD AS -85 DEGREES CELSIUS, THE OVERALL STRUCTURE HAS DEGRADED. THE MODERATE TO HIGH VERTICAL WIND SHEAR (VWS) OF 20-25 KTS HAS TAKEN A TOLL ON THE CONVECTIVE SIGNATURE, RESTRICTING UPPER-LEVEL OUTFLOW ON THE WESTERN SIDE OF THE SYSTEM. FURTHERMORE, TY 20W IS NOW TRAVERSING THE COOL WAKE LEFT BEHIND BY TROPICAL STORM (TS) 21W, WITH SEA SURFACE TEMPERATURES (SST) OF 27-28 DEGREES CELSIUS, A FEW DEGREES COOLER THAN 12 HOURS AGO. THE OCEAN, HOWEVER, IS STILL SUFFICIENTLY WARM TO MAINTAIN A TROPICAL CYCLONE. THEREFORE, THE ENVIRONMENT IS MARGINALLY UNFAVORABLE. A 290641Z RCM-1 SAR PASS ESTIMATED MAXIMUM SUSTAINED WINDS OF 80-85 KTS, WHICH WAS ASSESSED TO BE A SLIGHT OVERESTIMATE. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE MID-LEVEL CIRCULATION CENTER OBSERVED IN THE EIR IMAGERY. THE INITIAL INTENSITY IS HELD AT 75 KTS BASED ON THE EARLIER SAR DATA AND THE T4.5 FIXES FROM PGTW, PHFO, AND KNES. THIS ASSESSMENT IS NOTED TO BE WELL ABOVE THE CURRENT OBJECTIVE ESTIMATES FROM CIMSS. INITIAL WIND RADII BASIS: SAR DATA CURRENT STEERING MECHANISM: WESTERN FLANK OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED NEAR THE DATELINE AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T4.5 - 77 KTS RJTD: T4.0 - 65 KTS RCTP: T4.0 - 65 KTS KNES: T4.5 - 77 KTS PHFO: T4.5 - 77 KTS CIMSS SATCON: 59 KTS AT 290530Z CIMSS ADT: 51 KTS AT 291200Z CIMSS AIDT: 46 KTS AT 291200Z CIMSS D-MINT: 66 KTS AT 291113Z CIMSS D-PRINT: 61 KTS AT 291200Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 20-25 KTS SST: 27-28 CELSIUS OUTFLOW: MODERATE EASTWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE INTENSITY FORECAST HAS BEEN LOWERED 10-15 KTS ACROSS THE BOARD. FORECAST DISCUSSION: TY 20W WILL MAINTAIN A POLEWARD TRAJECTORY THROUGHOUT THE FORECAST PERIOD, STEERED BY AN EXTENSIVE RIDGE OVER THE CENTRAL NORTH PACIFIC. THE SYSTEM WILL TAKE A SLIGHT BEND NORTHWESTWARD FROM TAU 12 TO TAU 36, AS IT BRIEFLY INTERACTS WITH THE CIRCULATION OF TS 21W. BY TAU 48, AN APPROACHING LOW- TO MID-LEVEL TROUGH WILL INDUCE A NORTHEASTWARD MOTION. BASED ON THIS FORECAST TRACK, THE STORM WILL RAPIDLY WEAKEN. THE VWS WILL ENTRAIN INCREASING AMOUNTS OF DRY AIR INTO THE MINIATURE VORTEX. THE CIRCULATION WILL MOVE OVER PROGRESSIVELY COOLER SST, CROSSING THE 26 DEGREE ISOTHERM BY TAU 24. THESE FACTORS WILL RESULT IN A RAPID WEAKENING TREND AND A TRANSITION TO A SUBTROPICAL STORM BY TAU 48. MODEL DISCUSSION: MODELS REMAIN IN FAIR AGREEMENT THROUGH THE FORECAST, WITH BOTH A CROSS-TRACK AND ALONG-TRACK SPREAD OF APPROXIMATELY 140 NM AT TAU 48. THE GUIDANCE ENVELOPE THROUGH TAU 24 IS COMPRISED OF A TIGHT CORE OF PHYSICS-BASED AND AI MODELS WITH A FEW OUTLIERS ALONG THE WESTERN EDGE. THE JTWC TRACK FORECAST IS POSITIONED ALONG THIS CORE GROUP OF MODELS, WHICH LIES ALONG THE EASTERN SIDE OF THE ENVELOPE FOR THE FIRST 24 HOURS AND THEN LEANS TOWARD A BLEND OF THE AI CONSENSUS AND THE JTWC CONSENSUS (CONW). THE RELIABLE INTENSITY AIDS, INCLUDING HAFS-A AND GFS, DEPICT RAPID WEAKENING THROUGH TAU 48. THE JTWC INTENSITY FORECAST IS ALIGNED ACCORDINGLY, SET ALONG THE LOWER END OF THE SPREAD. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM INTENSITY 00-72 HR: MEDIUM// NNNN