MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (CHABA) WARNING NR 08// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 21W (CHABA), LOCATED APPROXIMATELY 996 NM SOUTHEAST OF KADENA AB, HAS TRACKED WESTWARD AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS AN AREA OF BROAD AND DEEP FLARING CONVECTION OVER THE LAST SIX HOURS THAT HAS RECENTLY EVOLVED INTO A BANDING FEATURE, DEFINITIVELY WRAPPING INTO THE LOW LEVEL CIRCULATION CENTER, GIVING HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY IS ASSESSED AT 45 KNOTS AND IS BASED ON THE IMPROVING CONVECTIVE STRUCTURE COMBINED WITH DVORAK ESTIMATES PGTW AND RJTD REPORTING T3.0 (45 KNOTS). UPPER-LEVEL ANALYSIS SHOWS IMPROVING RADIAL OUTFLOW, TAPPING INTO A STREAM OF STRONG WESTERLIES TO THE NORTH FLOWING INTO A FORMING TUTT CELL TO THE EAST. VERTICAL WIND SHEAR IS VERY LOW (5 TO 10 KNOTS) AND SSTS ARE FAVORABLE NEAR 30 CELSIUS. CURRENTLY TS CHABA IS TRACKING WEST-SOUTHWEST ALONG THE PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE POSITIONED TO THE NORTH. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. TS CHABA WILL CONTINUE ALONG THE EDGE OF THE CURRENT STEERING RIDGE TURNING NORTHWEST IN THE NEXT 12 TO 24 HOURS. ENVIRONMENTAL CONDITIONS WILL REMAIN FAVORABLE SUPPORTING STEADY INTENSIFICATION THROUGH TAU 72. DYNAMIC MODEL GUIDANCE IS IN RELATIVELY GOOD AGREEMENT FOR THIS PERIOD OF THE FORECAST GIVING HIGH CONFIDENCE TO THE FORECAST TRACK UP TO TAU 72. C. IN THE EXTENDED TAUS, A MID-LATITUDE TROUGH WILL ENTER THE REGION FROM THE WEST NEAR TAU 72 WEAKENING THE STEERING RIDGE. TS CHABA WILL BEGIN TO DECELERATE BRIEFLY AS IT ROUNDS THE RIDGE AXIS NEAR TAU 96, SHIFTING TO A NORTHEASTERN TRACK INTO THE WESTERLY FLOW AND ASSOCIATED TROUGH. HIGH VERTICAL WIND SHEAR AND LOWER SSTS WILL WEAKEN TS CHABA, AND EXTRATROPICAL TRANSITION WILL BEGIN NEAR TAU 120. MODEL GUIDANCE BECOMES MORE DIVERGENT BEYOND TAU 72 AS THE SYSTEM ROUNDS THE RIDGE AXIS. NAVGEM AND ECMWF ARE THE FAR WESTWARD SOLUTIONS SHOWING A WIDER AND SLOWER TURN AS A RESULT OF A WEAKER TROUGH AND PRONOUNCED STEERING RIDGE. NAVGEM AND ECMWF HAVE SHOWN INCONSISTENCY IN THEIR SOLUTIONS OVER THE PAST FEW CYCLES, WHILE GFS, COAMPS-TC, AND THE GFS ENSEMBLE HAVE BEEN FAIRLY CONSISTENT NEAR THE MULTI-MODEL CONSENSUS. DUE TO THE UNCERTAINTY IN MODEL TRACKERS BEYOND TAU 72, THERE IS LOW CONFIDENCE IN THE EXTENDED FORECAST TRACK.// NNNN NNNN