WDPN33 PGTW 010300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (ROKE) WARNING NR 012// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 31.8N 144.4E INITIAL INTENSITY: 55 KTS GEOGRAPHIC REFERENCE: 315 NM SOUTHEAST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 20 KTS SIGNIFICANT WAVE HEIGHT: 23 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN ASYMMETRIC CENTRAL DENSE OVERCAST THAT CONTINUES TO BE SHEARED NORTHEASTWARD ACROSS THE NORTHEASTERN AND EASTERN SEMICIRCLE, WITH A LOW LEVEL CIRCULATION THAT HAS BECOME OBSCURED BY DEEP CONVECTION. AN EARLIER 302114Z SSMIS 91GHZ MICROWAVE IMAGE REVEALS A DEEP CURVED CONVECTIVE BAND OVER THE NORTHWEST QUADRANT WRAPPING INTO A WELL DEFINED LOW LEVEL CIRCULATION. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE VISIBLE IMAGERY ALONG WITH THE AFOREMENTIONED SSMIS MICROWAVE IMAGE. 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, SATCON, AND DMN. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: NORTHWEST PERIPHERY OF MID-LEVEL SUBTROPICAL RIDGE (STR) AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.5 - 55 KTS RJTD: T3.0 - 45 KTS CIMSS SATCON: 61 KTS AT 301642Z CIMSS ADT: 79 KTS AT 302340Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 25-30 KTS SST: 27-28 CELSIUS OUTFLOW: STRONG POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM 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: TS 20W IS CURRENTLY ACCELERATING NORTHEASTWARD AS THE SYSTEM TRANSITS ALONG THE NORTHWEST PERIPHERY OF THE MID-LEVEL SUBTROPICAL RIDGE (STR) TO THE EAST-SOUTHEAST. THE SYSTEM WILL CONTINUE TO TRANSIT NORTHEASTWARD QUICKLY OVER THE NEXT 24 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 48. BY TAU 24, A SUBTROPICAL SHORTWAVE TROUGH WILL INTERACT WITH THE SYSTEM, WHICH WILL BEGIN INITIATING SUBTROPICAL TRANSITION, AND IS FORECAST TO COMPLETE SUBTROPICAL TRANSITION BY TAU 48. AS THIS SUBTROPICAL TRANSITION OCCURS THE SYSTEMS WIND FIELD WILL ALSO EXPAND INTO A MORE BROAD GALE FORCE WIND FIELD. TS 20W IS EXPECTED TO MAINTAIN ITS CURRENT INTENSITY BEFORE A STEADY WEAKENING TREND OCCURS AS THE SYSTEM TRANSITS IN A MARGINALLY FAVORABLE ENVIRONMENT. THESE MARGINALLY FAVORABLE ENVIRONMENTAL FACTORS ARE CHARACTERIZED BY WARM (28-29C) SEA SURFACE (SSTS), STRONG POLEWARD OUTFLOW, OFFSET BY HIGH (25-30KTS) VERTICAL WIND SHEAR. MODEL DISCUSSION: NUMERICAL MODELS ARE IN TIGHT AGREEMENT WITH A GRADUAL CROSS TRACK SPREAD OF 150 NM BY TAU 48. THIS SPREAD IS RELATIVELY LOW EVEN THOUGH THE SYSTEM WILL BE INFLUENCED BY A RELATIVELY COMPLEX SYNOPTIC STEERING PATTERN. BOTH THE 301800Z GFS ENSEMBLE (GEFS) AND 301800Z ECMWF ENSEMBLE (EPS) ARE IN GOOD AGREEMENT ON THE NORTHEAST TRACK FORECAST ALONG WITH A SLOWED STORM MOTION BY THE END OF THE FORECAST BY TAU 48. THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC TRACK FORECAST THROUGH TAU 48. THERE IS MEDIUM CONFIDENCE IN THE JTWC INTENSITY FORECAST AND FOLLOWS CLOSE TO THE MULTI-MODEL CONSENSUS. AS GFS AND COAMPS-TC BOTH SHOW A PEAK INTENSITY OF 65 KTS, WITH GFS DECAY SHIPS (DSHA) REPRESENTING THE LOW END OF INTENSITY GUIDANCE WITH A STEADY DECREASE FROM THE INITIAL INTENSITY THROUGH OUT THE FORECAST. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH INTENSITY 0 - 72 HR: MEDIUM// NNNN