WDPN33 PGTW 292100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (ROKE) WARNING NR 007// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 27.2N 134.1E INITIAL INTENSITY: 75 KTS GEOGRAPHIC REFERENCE: 564 NM SOUTH-SOUTHWEST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 10 KTS SIGNIFICANT WAVE HEIGHT: 28 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TY 20W IS ROUNDING THE SUBTROPICAL RIDGE AXIS AND IS STEADILY WEAKENING AS IT ENCOUNTERS THE MIDLATITUDE WESTERLIES AND INCREASING VERTICAL WIND SHEAR (25 TO 30 KNOTS). ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A MORE ASYMMETRIC CENTRAL DENSE OVERCAST WITH WARMING CLOUD TOP TEMPERATURES REFLECTING THE GENERAL WEAKENING TREND. A 291628Z AMSR2 89GHZ MICROWAVE IMAGE REVEALS TIGHTLY-CURVED BANDING WRAPPING INTO A WELL-DEFINED LOW-LEVEL CIRCULATION CENTER WITH THE BULK OF THE DEEP CONVECTION SHEARED OVER THE EASTERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE TIMELY AMSR2 MICROWAVE IMAGE. THE INITIAL INTENSITY OF 75 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE KNES AND RJTD DVORAK CURRENT INTENSITY ESTIMATES AS WELL AS THE CIMSS ADT ESTIMATE. INITIAL WIND RADII BASIS: WIND RADII HAVE BEEN ADJUSTED BASED ON AMSR2 WINDSPEED IMAGE, WHICH SHOWED A COMPACT, SYMMETRIC WINDFIELD. CURRENT STEERING MECHANISM: NORTHWEST PERIPHERY OF A LOW- TO MID-LEVEL SUBTROPICAL RIDGE (STR). AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T5.5 - 102 KTS RJTD: T4.5 - 77 KTS KNES: T4.5 - 77 KTS CIMSS SATCON: 59 KTS AT 291703Z CIMSS ADT: 74 KTS AT 291740Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 25-30 KTS SST: 28-29 CELSIUS OUTFLOW: STRONG POLEWARD OTHER FACTORS: DRY AIR ENTRAINMENT ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: INTRODUCED SUBTROPICAL TRANSITION IN THE EXTENDED FORECAST PERIOD. FORECAST DISCUSSION: TY 20W IS FORECAST TO TRACK EAST-NORTHEASTWARD THROUGH TAU 48 ALONG THE NORTHERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) WITHIN THE MIDLATITUDE WESTERLIES. THIS IS EVIDENCED IN THE 291200Z SOUNDING FROM MINAMIDAITO-JIMA AND THE LATEST CIMSS UPPER-LEVEL WIND IMAGE, WHICH SHOW STRONG WESTERLIES OVER THE EAST CHINA SEA AND JAPAN WITH A JET DIPPING DOWN OVER THE NORTHERN RYUKYU ISLANDS. DUE TO THE SYSTEM'S POSITION UNDER THE SUBTROPICAL WESTERLIES, TRACK SPEEDS WILL BE FAIRLY QUICK (13-14 KNOTS) THROUGH TAU 48, FOLLOWED BY A SIGNIFICANT SLOW-DOWN THROUGH TAU 96, WHICH IS REFLECTED IN THE NUMERICAL MODEL GUIDANCE. THE PRIMARY REASON FOR THE SLOW-DOWN IS A LOW-LEVEL HIGH THAT BUILDS IN TO THE EAST AND NORTHWEST THROUGH TAU 96, EFFECTIVELY BLOCKING OR SLOWING POLEWARD PROGRESSION FOR ABOUT 2 DAYS. AFTER TAU 96, A DEEP, DYNAMIC SUBTROPICAL SHORTWAVE TROUGH WILL APPROACH AND ACCELERATE THE SYSTEM NORTHEASTWARD WHILE INITIATING A SUBTROPICAL TRANSITION. DUE TO THE LOCATION OF THE POLAR FRONT WELL TO THE NORTH OF JAPAN, THE SYSTEM WILL NOT UNDERGO EXTRA-TROPICAL TRANSITION AND WILL NOT ACCELERATE LIKE A TYPICAL EXTRA-TROPICAL SYSTEM. TY 20W IS EXPECTED TO WEAKEN STEADILY THROUGH THE FORECAST PERIOD UNDER THE MARGINAL CONDITIONS. MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE HAS BEEN STRUGGLING RUN-TO-RUN WITH THIS COMPLEX, EVOLVING SYNOPTIC PATTERN. THE CURRENT RUN IS IN FAIR CROSS-TRACK AGREEMENT THROUGH THE FORECAST PERIOD. HOWEVER, THERE ARE SOME DIFFERENCES IN ALONG-TRACK SPEEDS. MOST OF THE MODELS SHOW A SLOW-DOWN (LESS THAN 5 KNOTS) BY TAU 96 WITH A PERIOD OF SHARP ACCELERATION THROUGH TAU 120. THE LATEST ECMWF TRACKER IS THE FASTEST OUTLIER AND SHOWS 11 KNOT TRACK SPEEDS THROUGH TAU 96 THEN AN INCREASE TO 18 KNOTS BY TAU 120. THE 291200Z ECMWF ENSEMBLE (EPS) SHOWS SOME OF THE SOLUTIONS SLOWING BUT HAS TRENDED TOWARD AN INCREASING NUMBER OF FASTER SOLUTIONS. THE GFS ENSEMBLE (GEFS) FAVORS SLOWER TRACK SPEEDS IN THE TAU 72 TO TAU 96 PERIOD THEN AN ACCELERATED TRACK AFTER TAU 96. CONSEQUENTLY, THERE IS LOW CONFIDENCE IN THE JTWC TRACK FORECAST AFTER TAU 72. THERE IS ALSO LOW CONFIDENCE IN THE JTWC INTENSITY FORECAST AFTER TAU 72 WITH DISSIPATION POSSIBLE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN