MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (CHABA) WARNING NR 28// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 21W (CHABA), LOCATED APPROXIMATELY 160 NM WEST- SOUTHWEST OF SASEBO, JAPAN, HAS TRACKED NORTH-NORTHEASTWARD AT 15 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS A RAPIDLY DETERIORATING SYSTEM WITH DEEP CONVECTION SHEARED OVER THE NOTHERN SEMI-CIRCLE OF A PARTIALLY-EXPOSED LOW- LEVEL CIRCULATION CENTER (LLCC). HOWEVER, KMA RADAR IMAGERY SHOWS A PARTIAL EYEWALL OVER THE NORTH QUADRANT OF THE LLCC AND ALSO A BROAD CENTER JUST SOUTH TO SOUTHEAST OF CHEJU ISLAND. RADAR ANIMATION INDICATES THAT THE CENTER IS CURRENTLY PASSING SOUTH OF THE ISLAND AS IT RE-CURVES NORTHEASTWARD. SURFACE OBSERVATIONS FROM CHEJU INTERNATIONAL AIRPORT (RKPC), 60NM NORTH OF THE 04/18Z CENTER POSITION, REVEAL SUSTAINED NORTH-NORTHEASTERLY WINDS OF 45 KNOTS GUSTING TO 68 KNOTS ASSOCIATED WITH THE EYEWALL CONVECTION. SURFACE OBSERVATIONS FROM WESTERN KYUSHU, AHEAD OF THE SYSTEM, ARE STILL RELATIVELY LOW AT 20 TO 30 KNOTS BUT EXPECTED TO INCREASE. A 041755Z SSMI 85GHZ IMAGE SHOWS A DEVELOPING WEAK COLD FRONT MOVING OVER WESTERN KYUSHU, WHICH IS INDICATIVE OF THE EARLY STAGES OF EXTRA- TROPICAL TRANSITION (ETT). OVERALL, THERE IS GOOD CONFIDENCE IN THE CURRENT POSITION AND RECENT TRACK MOTION CHANGE BASED ON THE AVAILABLE RADAR IMAGERY. THE CURRENT INTENSITY IS ASSESSED AT 85 KNOTS BASED ON THE LOWER RANGE OF DVORAK INTENSITY ESTIMATES AND THE WEAKENED STRUCTURE. UPPER-LEVEL ANALYSIS INDICATES STRONG SOUTHWESTERLY VERTICAL WIND SHEAR (VWS) OFFSET SLIGHTLY BY ENHANCED POLEWARD OUTFLOW. TY CHABA IS TRACKING NORTH-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. THIS EXTRATROPICAL LOW IS THEN FORECAST TO CROSS HONSHU, WHICH WILL SERVE TO FURTHER WEAKEN THE SYSTEM. DYNAMIC GUIDANCE REMAINS IN TIGHT AGREEMENT, THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN