MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (CHABA) WARNING NR 25// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 21W (CHABA), LOCATED APPROXIMATELY 361 NM SOUTH- SOUTHWEST OF SASEBO, JAPAN, HAS TRACKED NORTH-NORTHWESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY INDICATES A SLIGHT WEAKENING TREND WITH A 10-NM CLOUD-FILLED EYE, WHICH PROVIDES GOOD CONFIDENCE IN THE CURRENT POSITION. HOWEVER, A 032238Z SSMIS 91GHZ IMAGE AND RADAR IMAGERY SHOW A COMPACT CORE OF DEEP CONVECTION WITH A 70-NM DIAMETER EYEWALL AND DEEP CONVECTIVE BANDING OVER THE NORTHERN SEMI-CIRCLE. A ROBUST POLEWARD OUTFLOW AND VERY WARM SEA SURFACE TEMPERATURES (SSTS) ARE COMBINING TO FUEL THE STORM AND SUSTAIN THE CURRENT INTENSITY. THE CURRENT INTENSITY IS ASSESSED AT 125 KNOTS BASED ON DVORAK ESTIMATES RANGING FROM 5.5 (102 KNOTS) TO 7.5 (155 KNOTS). SOUTHWESTERLY VERTICAL WIND SHEAR (VWS) HAS INCREASED TO 25-35 KNOTS AND IS RESPONSIBLE FOR THE RECENT WEAKENING OF THE CONVECTIVE STRUCTURE. TY 21W IS TRACKING NORTHWARD ALONG THE WESTERN PERIPHERY OF A DEEP- LAYERED SUBTROPICAL RIDGE (STR) TO THE EAST. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY CHABA WILL CONTINUE TRACKING NORTHWARD OVER THE NEXT 12 HOURS AND THEN NORTHEASTWARD AS IT ROUNDS THE STR AXIS. THE SYSTEM WILL ENCOUNTER INCREASING VWS AS IT MOVES CLOSER TO THE STRONG ZONAL WESTERLY FLOW SITUATED JUST SOUTH OF THE KOREAN PENINSULA. THIS WILL FORCE THE SYSTEM TO FURTHER ELONGATE TO THE NORTHEAST AND RAPIDLY WEAKEN. TY 21W WILL BEGIN EXTRA-TROPICAL TRANSITION (ETT) NEAR TAU 24 AS IT EMBEDS WITHIN THE MID-LATITUDE WESTERLY FLOW ALONG THE WESTERN EDGE OF MAINLAND JAPAN, AND WILL COMPLETE ETT BY TAU 48. IT WILL CONTINUE TO WEAKEN AS IT TRACKS OVER THE COOL WATERS IN THE SEA OF JAPAN AND THEN ACROSS THE RUGGED JAPANESE TERRAIN. DYNAMIC MODEL GUIDANCE IS IN TIGHT AGREEMENT THROUGHOUT THE FORECAST. ADDITIONALLY, GLOBAL MODEL FIELDS SHOW NO CHANGE TO THE LOCATION OF THE MID-LATITUDE WESTERLIES; THEREFORE, ONLY MINIMAL CHANGES TO THE FORECAST TRACK ARE EXPECTED. GIVEN THESE FACTORS, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST.// NNNN NNNN