WDPN33 PGTW 302100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (ROKE) WARNING NR 011// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 30.6N 142.5E INITIAL INTENSITY: 55 KTS GEOGRAPHIC REFERENCE: 315 NM SOUTH-SOUTHEAST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 18 KTS SIGNIFICANT WAVE HEIGHT: 23 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS AN ASYMMETRIC CENTRAL DENSE OVERCAST, WITH A LOW LEVEL CIRCULATION THAT HAS BECOME INCREASINGLY EXPOSED AS THE BULK OF THE CONVECTION IS SHEARED NORTHEASTWARD ACROSS THE EASTERN SEMICIRCLE. AN EARLIER 301325Z GMI 89GHZ MICROWAVE IMAGE REVEALS CURVED CONVECTIVE BANDING WRAPPING INTO A WELL DEFINED LOW LEVEL CIRCULATION. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE EIR IMAGERY AS WELL AS A PARTIAL 301111Z METOP-C ASCAT. THE INITIAL INTENSITY OF 55 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A COMBINATION OF PGTW AND RJTD DVORAK CURRENT INTENSITY ESTIMATES ALONG WITH OBJECTIVE DVORAK ESTIMATES FROM CIMSS ADT AND SATCON. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: NORTHEAST PERIPHERY OF MID-LEVEL SUBTROPICAL RIDGE (STR) AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS RJTD: T3.5 - 55 KTS CIMSS SATCON: 66 KTS AT 301235Z CIMSS ADT: 79 KTS AT 301740Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 25-30 KTS SST: 28-29 CELSIUS OUTFLOW: STRONG POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: CURRENT ENVIRONMENTAL FACTORS ARE MARGINALLY FAVORABLE CHARACTERIZED BY WARM (28-29C) SEA SURFACE (SSTS), STRONG POLEWARD OUTFLOW, OFFSET BY HIGH (25-30KTS) VERTICAL WIND SHEAR. TS 20W IS CURRENTLY ACCELERATING NORTHEASTWARD AS THE SYSTEM TRANSITS ALONG THE NORTHWEST PERIPHERY OF THE MID-LEVEL SUBTROPICAL RIDGE (STR) TO THE SOUTHEAST. THE SYSTEM WILL CONTINUE TO TRANSIT NORTHEASTWARD QUICKLY OVER THE NEXT 36 HOURS, DUE TO THE SYSTEM'S POSITION WITHIN THE SUBTROPICAL WESTERLIES. AFTER WHICH, TS 20W'S ACCELERATED TRACK MOTION IS FOLLOWED BY A SIGNIFICANT DECELERATION AND SLOWED POLEWARD PROGRESSION, AS A LOW-LEVEL HIGH BUILDS IN TO THE NORTH-NORTHWEST, THROUGH TAU 72. BY TAU 36, A SUBTROPICAL SHORTWAVE TROUGH WILL INTERACT WITH THE SYSTEM, WHICH WILL BEGIN INITIATING SUBTROPICAL TRANSITION, AND IS FORECAST TO COMPLETE SUBTROPICAL TRANSITION BY TAU 72. AS THIS SUBTROPICAL TRANSITION OCCURS THE SYSTEMS WIND FIELD WILL ALSO EXPAND INTO A MORE BROAD GALE FORCE WIND FIELD. TY 20W IS EXPECTED TO MAINTAIN ITS CURRENT INTENSITY BEFORE A STEADY WEAKENING TREND OCCURS AS THE SYSTEM TRANSITS IN A MARGINALLY FAVORABLE ENVIRONMENT. MODEL DISCUSSION: NUMERICAL MODELS ARE IN TIGHT AGREEMENT WITH A GRADUAL CROSS TRACK SPREAD OF 130 NM BY TAU 72. THIS UNCERTAINLY IN THE TRACK FORECAST IS DUE TO THE COMPLEX STEERING BEYOND TAU 36. HOWEVER, THE 301200Z GFS ENSEMBLE (GEFS) AND 300600Z ECMWF ENSEMBLE (EPS) BOTH AGREE ON THE NORTHEAST TRACK FORECAST ALONG WITH A SLOWED STORM MOTION BY THE END OF THE FORECAST. THEREFORE, THERE IS MEDIUM CONFIDENCE IN THE JTWC TRACK FORECAST THROUGH TAU 72. THERE IS MEDIUM CONFIDENCE IN THE JTWC INTENSITY FORECAST AND FOLLOWS CLOSE TO THE MULTI-MODEL CONSENSUS. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM// NNNN