MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (CHABA) WARNING NR 29// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 21W (CHABA), LOCATED APPROXIMATELY 100 NM NORTHWEST OF SASEBO, JAPAN, HAS TRACKED NORTHEASTWARD AT 20 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A RAPIDLY DETERIORATING SYSTEM WITH AN EXPOSED, BROADENING LOW-LEVEL CIRCULATION CENTER (LLCC) LOCATED ON THE SOUTHWEST EDGE OF THE DECAYING DEEP CONVECTION. THE CENTER HAS TRACKED JUST SOUTH OF BUSAN, KOREA, WHICH IS REPORTING EAST-NORTHEASTERLY TO NORTHEASTERLY SUSTAINED WINDS OF 45 KNOTS WITH GUSTS UP TO 55 KNOTS. RECENT JMA RADAR IMAGERY SHOWS A FRONTAL-LIKE CONVECTIVE PATTERN, AND PROVIDES EVIDENCE THAT THE SYSTEM IS UNDERGOING EXTRA-TROPICAL TRANSITION (ETT). THE CURRENT POSITION IS BASED ON RADAR FIXES AND A 042156Z WINDSAT 37GHZ MICROWAVE EYE FEATURE. THE CURRENT INTENSITY IS ASSESSED AT 75 KNOTS BASED ON THE DVORAK CURRENT INTENSITY ESTIMATES OF 77 KNOTS (4.5) FROM PGTW AND KNES. UPPER-LEVEL ANALYSIS INDICATES STRONG SOUTHWESTERLY VERTICAL WIND SHEAR (VWS) OFFSET SLIGHTLY BY ENHANCED POLEWARD OUTFLOW. TY CHABA IS TRACKING NORTHEASTWARD ALONG THE NORTHWEST PERIPHERY OF THE DEEP-LAYERED SUBTROPICAL RIDGE (STR). 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY CHABA WILL CONTINUE TO ACCELERATE NORTHEASTWARD AS IT ROUNDS THE STR AXIS AND ENTERS THE MIDLATITUDE WESTERLIES. SIGNIFICANT WEAKENING IS EXPECTED TO OCCUR DUE TO COOLER SSTS AND INCREASING VWS. THE SYSTEM IS EXPECTED TO COMPLETE ETT BY TAU 24 AS IT GAINS FRONTAL CHARACTERISTICS AND BECOMES EMBEDDED WITHIN THE MIDLATITUDE WESTERLIES. DYNAMIC GUIDANCE REMAINS IN TIGHT AGREEMENT, THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN