MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (CHABA) WARNING NR 09// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 21W (CHABA), LOCATED APPROXIMATELY 971 NM SOUTHEAST OF KADENA AB, HAS TRACKED WESTWARD AT 10 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS AN AREA OF CONSOLIDATING CONVECTION WITH FORMATIVE BANDING WRAPPING INTO THE CENTER. THE INITIAL POSITION IS BASED ON MULTI-AGENCY SATELLITE FIXES AND A 292150Z SSMIS 91 GHZ IMAGE SHOWING FRAGMENTED BANDS OF DEEP CONVECTION CONFINED TO THE SOUTHWESTERN AND SOUTHEASTERN PERIPHERIES AND WRAPPING INTO THE LOW LEVEL CIRCULATION CENTER. THE INITIAL INTENSITY IS ASSESSED AT 50 KNOTS AND IS BASED ON AN AVERAGE OF MULTI-AGENCY DVORAK ESTIMATES RANGING FROM T3.0 TO T3.5 (45 TO 55 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 WESTWARD 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, HOWEVER MINOR TRACK CHANGES REFLECT A SLOWER TRACK SPEED AND SLIGHTLY WIDER TURN IN THE EXTENDED FORECAST. B. TS CHABA WILL CONTINUE ALONG THE EDGE OF THE CURRENT STEERING RIDGE TURNING NORTHWEST IN THE NEXT 12 TO 18 HOURS. ENVIRONMENTAL CONDITIONS WILL REMAIN FAVORABLE, SUPPORTING STEADY INTENSIFICATION THROUGH 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. DYNAMIC MODEL GUIDANCE IS SHOWING INCREASED DIVERGENCE IN BOTH THE NEAR-TERM TRACK AND THE TURN TO THE NORTHEAST. NAVGEM AND GFDN ARE THE FAR WESTWARD OUTLIERS SHOWING A MUCH WIDER TURN TO THE NORTHEAST. THE UKMET AND HWRF SOLUTIONS ARE THE FAR EASTWARD OUTLIERS SHOWING A TIGHTER TURN. THE GFS, ECMWF, AND COAMPS-TC SOLUTIONS FALL NEAR THE MULTI-MODEL CONSENSUS AND IS WHERE THE FORECAST TRACK IS PLACED. DUE TO THE MODEL DIVERGENCE, THERE IS LOW CONFIDENCE IN THE FORECAST TRACK.// NNNN NNNN