MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (CHABA) WARNING NR 20// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON 21W (CHABA), LOCATED APPROXIMATELY 224 NM SOUTH OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTHWESTWARD AT 14 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED IMAGERY (EIR) SHOWS THE SYSTEM HAS MAINTAINED A RAGGED 15-NM EYE AS THE CENTRAL CONVECTION DEEPENED SLIGHTLY. THE INITIAL POSITION IS PLACED WITH CONFIDENCE BASED ON EYE IN THE EIR LOOP. THE INITIAL INTENSITY OF 115 KNOTS IS BASED ON CONGRUENT DVORAK ESTIMATES OF T6.0 FROM 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 21W 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, BRUSHING THE NORTHERN COASTAL AREAS OF JAPAN. BY TAU 72, THE CYCLONE WILL BE OVER NORTH-CENTRAL HONSHU NORTH OF TOKYO. THE ENVIRONMENT WILL REMAIN FAVORABLE FOR FURTHER DEVELOPMENT OVER THE NEXT 12 HOURS - ENHANCED BY INCREASED POLEWARD OUTFLOW - LEADING TO A PEAK OF 125 KNOTS. AFTER TAU 12, INCREASING VWS, THEN LAND INTERACTION WITH THE RUGGED TERRAIN OF JAPAN WILL CAUSE GRADUAL THEN RAPID DECAY. BY TAU 72, TY 21W WILL BE REDUCED TO 40 KNOTS, POSSIBLY WEAKER. CONCURRENTLY, THE SYSTEM WILL UNDERGO EXTRA-TROPICAL TRANSITION (ETT) AS IT GETS EMBEDDED IN THE BAROCLINIC ZONE. C. BEYOND TAU 72, TY CHABA WILL CONTINUE TO ACCELERATE BUT MORE ON AN EAST-NORTHEASTWARD TRAJECTORY, EXIT JAPAN INTO THE PACIFIC OCEAN, AND COMPLETE ETT BY TAU 96. DYNAMIC MODEL GUIDANCE IS IN BETTER AGREEMENT, BUT 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