MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (CHABA) WARNING NR 21// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON 21W (CHABA), LOCATED APPROXIMATELY 157 NM SOUTH OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTHWESTWARD AT 13 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THE SYSTEM HAS REGAINED SYMMETRY AND A SHARPLY OUTLINED 7-NM EYE AS THE CENTRAL CONVECTION FURTHER DEEPENED. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE EYE IN THE MSI LOOP. THE INITIAL INTENSITY OF 125 KNOTS IS BASED ON CONGRUENT DVORAK ESTIMATES OF T6.0 FROM SATCON AND ALL REPORTING AGENCIES. ENVIRONMENTAL CONDITIONS REMAIN FAVORABLE WITH WARM (30C) SSTS, LOW TO MODERATE (10-15 KNOT) VERTICAL WIND SHEAR (VWS), AND EXCELLENT RADIAL OUTFLOW WITH A STRONG BIAS ON THE POLEWARD CHANNEL. THE CYCLONE IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR). 3. FORECAST REASONING. A. NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. TY CHABA WILL TRACK NORTHWEST THEN NORTHWARD OVER THE NEXT 24 HOURS. AFTERWARD, THE SYSTEM WILL ROUND THE STEERING STR AND BEGIN TO TRACK AND ACCELERATE NORTHEASTWARD THROUGH THE TSUSHIMA STRAIT INTO THE SEA OF JAPAN (SOJ), THEN ACROSS NORTHERN HONSHU BEFORE EXITING INTO THE NORTHWEST PACIFIC OCEAN EAST OF MISAWA BEFORE TAU 72. THE ENVIRONMENT WILL REMAIN FAVORABLE FOR FURTHER DEVELOPMENT OVER THE NEXT 12 HOURS - ENHANCED BY INCREASED POLEWARD OUTFLOW - LEADING TO A PEAK OF 135 KNOTS. AFTER TAU 12, INCREASING VWS, COLDER SSTS IN THE SOJ, THEN LAND INTERACTION WITH THE RUGGED TERRAIN OF NORTHERN HONSHU WILL CAUSE GRADUAL THEN RAPID DECAY. BY TAU 48, THE SYSTEM WILL UNDERGO EXTRA-TROPICAL TRANSITION (ETT) AS IT GETS EMBEDDED IN THE BAROCLINIC ZONE. BY TAU 72, TY 21W WILL BE REDUCED TO 35 KNOTS, POSSIBLY WEAKER, AS A COLD CORE LOW. DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT; HOWEVER, THE TIMING OF THE RECURVE TOWARD THE NORTHEAST AND EVENTUAL LANDFALL IS STILL UNCERTAIN. THEREFORE, THERE IS LOW CONFIDENCE IN THE JTWC TRACK FORECAST BEYOND TAU 24.// NNNN NNNN