MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (CHABA) WARNING NR 12// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (CHABA), LOCATED APPROXIMATELY 809 NM SOUTHEAST OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTHWESTWARD AT 09 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY INDICATES A COMPACT, CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC) WITH FRAGMENTED CONVECTIVE BANDING. THE INITIAL POSITION IS BASED ON A LOW REFLECTIVITY CIRCULATION FEATURE IN THE 301722Z MICROWAVE PASS WITH GOOD CONFIDENCE. THE INITIAL INTENSITY OF 50 KNOTS IS BASED ON AN OVERALL ASSESSMENT OF OBJECTIVE AND AGENCY DVORAK ESTIMATES AND REFLECTS THE SLIGHT, ALBEIT MOMENTARY DEGRADATION IN THE SYSTEM. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM IS IN AN AREA OF LOW TO MODERATE (10-15 KNOT) VERTICAL WIND SHEAR (VWS) AND GOOD RADIAL OUTFLOW. TS 21W IS TRACKING ALONG THE SOUTHWEST PERIPHERY OF THE DEEP-LAYERED SUBTROPICAL RIDGE (STR). 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. TS CHABA WILL CONTINUE TO TRACK NORTHWESTWARD ALONG THE SOUTHWEST PERIPHERY OF THE DEEP-LAYERED STR WHILE INTENSIFYING STEADILY DUE TO OVERALL FAVORABLE ENVIRONMENTAL CONDITIONS. AFTER TAU 48, A MID-LATITUDE TROUGH APPROACHING FROM THE NORTHWEST WILL WEAKEN THE STR AND CAUSE THE CYCLONE TO TRACK MORE NORTHWARD. TS 21W IS EXPECTED TO PEAK AT 90 KNOTS NEAR TAU 72. C. AFTER TAU 72, TS CHABA WILL ACCELERATE NORTHEASTWARD, MAKE LANDFALL OVER SOUTHERN KYUSHU, JAPAN, AND DRAG ACROSS SOUTHERN JAPAN. INCREASING VWS, IN ADDITION TO LAND INTERACTION, WILL RAPIDLY ERODE THE SYSTEM; DOWN TO 40 KNOTS BY TAU 120. CONCURRENTLY, THE SYSTEM WILL BEGIN EXTRA-TROPICAL TRANSITION. NUMERIC MODEL GUIDANCE IS IN TIGHT AGREEMENT UP TO TAU 72 WHERE THE CONSENSUS MEMBERS SIGNIFICANTLY DIVERGE IN BOTH DIRECTION AND SPEED. THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC TRACK FORECAST UP TO TAU 72, THEN LOW CONFIDENCE AFTERWARDS. NNNN NNNN