MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (CHABA) WARNING NR 17// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 21W (CHABA), LOCATED APPROXIMATELY 462 NM SOUTH-SOUTHEAST OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTHWESTWARD AT 16 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THE SYSTEM HAS RAPIDLY DEEPENED AND BECOME MORE SYMMETRICAL AS FEEDER BANDS WRAPPED AND COALESCED INTO THE SUPER-DEEP CENTRAL CONVECTION. THE INITIAL POSITION IS BASED ON A SHARPLY OUTLINED 5-NM EYE IN THE MSI LOOP WITH HIGH CONFIDENCE. THE INITIAL INTENSITY OF 105 KNOTS IS BASED ON AGENCY DVORAK ESTIMATES RANGING FROM T5.5 TO T6.0 AND REFLECTS THE RAPID INTENSIFICATION. TY 21W IS TRACKING IN A VERY FAVORABLE ENVIRONMENT WITH STRONG DIFFLUENCE ALOFT, LOW (05-10 KNOT) VERTICAL WIND SHEAR (VWS), AND VERY WARM (30C) SEA SURFACE TEMPERATURES (SSTS). THE CYCLONE IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF THE DEEP-LAYERED SUBTROPICAL RIDGE (STR). 3. FORECAST REASONING. A. THE FORECAST PEAK INTENSITY HAS BEEN RAISED TO 130 KNOTS AND THE TRACK HAS BEEN WIDENED FURTHER WEST OF KADENA AB AND FURTHER NORTH ALONG THE JAPANESE ISLANDS. OTHERWISE, NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. TY 21W WILL CONTINUE NORTHWEST TO NORTH-NORTHWESTWARD OVER THE NEXT 36 HOURS TOWARDS OKINAWA. AFTERWARDS, IT WILL ROUND THE STR AXIS AND RE-CURVE TO THE NORTHEAST. ENVIRONMENTAL CONDITIONS WILL REMAIN FAVORABLE FOR CONTINUED INTENSIFICATION IN THE NEAR TERM, PEAKING AT 135 KNOTS BY TAU 36 PARTLY DUE TO AN APPROACHING MID-LATITUDE TROUGH THAT WILL FURTHER ENHANCE POLEWARD OUTFLOW. JUST BEFORE TAU 72, THE CYCLONE WILL MAKE LANDFALL OVER NORTHERN KYUSHU, JAPAN NEAR SASEBO AND BEGIN TO WEAKEN MOSTLY DUE TO INCREASING VWS AND LAND INTERACTION. C. IN THE EXTENDED FORECAST, TY CHABA WILL ACCELERATE NORTHEASTWARD AND DRAG ACROSS THE RUGGED JAPANESE ISLAND CHAIN AND RAPIDLY ERODE DUE TO LAND INTERACTION IN ADDITION TO STRONG (30-40 KNOT) VWS. CONCURRENTLY, THE SYSTEM WILL UNDERGO EXTRA-TROPICAL TRANSITION (ETT) AND IS EXPECTED TO COMPLETE ETT BY THE END OF THE FORECAST PERIOD AFTER IT TRACKS BACK INTO THE PACIFIC OCEAN SOUTH OF MISAWA. DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT UP TO TAU 36, AFTERWARDS, IT SPREADS SIGNIFICANTLY BOTH IN TERMS OF SPEED AND DIRECTION. IN VIEW OF THIS, THERE IS LOW CONFIDENCE IN THE JTWC TRACK FORECAST BEYOND TAU 36.// NNNN NNNN