MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (CHABA) WARNING NR 06// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (CHABA), LOCATED APPROXIMATELY 198 NM WEST-NORTHWEST OF NAVSTA GUAM, HAS TRACKED WESTWARD AT 14 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS IMPROVED OVERALL CONVECTIVE STRUCTURE, WITH BANDING BEGINNING TO WRAP INTO THE LOW-LEVEL CIRCULATION CENTER (LLCC). A 290603Z SSMIS 91 GHZ IMAGE REVEALS THE LLCC IS POSITIONED ALONG THE NORTHERN PERIPHERY OF THE DEEPEST CONVECTION, AND SUPPORTS THE CURRENT BEST TRACK POSITION WITH FAIR CONFIDENCE. THE INTENSITY IS ASSESSED AT 35 KNOTS BASED ON DVORAK CURRENT INTENSITY ESTIMATES OF T2.5 (35 KNOTS) FROM PGTW AND RJTD. UPPER-LEVEL ANALYSIS INDICATES A MORE FAVORABLE ENVIRONMENT FOR DEVELOPMENT WITH IMPROVING OUTFLOW AND LOW (10 TO 15 KNOTS) VERTICAL WIND SHEAR (VWS). TS CHABA IS TRACKING GENERALLY WESTWARD ALONG THE SOUTHERN PERIPHERY OF A DEEP LAYERED SUB-TROPICAL RIDGE (STR) TO THE NORTH. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. THE STR WILL STEER TS CHABA TOWARD THE WEST-NORTHWEST WITH A GRADUAL TURN TOWARD THE NORTHWEST, AS THE UPPER-LEVEL FEATURE RE- ORIENTS. STEADY INTENSIFICATION WILL COMMENCE AS POLEWARD OUTFLFOW CONTINUES TO IMPROVE IN RESPONSE TO A WEAKENING TUTT CELL TO THE WEST OF THE SYSTEM. TS CHABA IS FORECAST TO REACH TYPHOON INTENSITY BETWEEN TAU 36 AND 48. DYNAMIC MODEL GUIDANCE REMAIN IN GOOD AGREEMENT ON THE TRACK OF THE SYSTEM IN THE NEAR TERM AND THE OFFICIAL JTWC FORECAST TRACK REMAINS NEAR THE MULTI-MODEL CONSENSUS. C. IN THE EXTENDED PERIOD, DYNAMIC MODEL GUIDANCE DIVERGES GREATLY BEYOND TAU 72, WITH THE GFS AND JGSM REMAINING ON THE RIGHT SIDE OF THE MODEL CONSENSUS, WITH THE NAVGEM AND ECMFW TO THE LEFT OF CONSENSUS. AT THIS TIME, THE OFFICIAL FORECAST TRACK REMAINS TO THE LEFT OF THE MULTI-MODEL CONSENSUS FROM TAU 72 THROUGH 120 BASED ON RUN-TO-RUN CONSISTENCY FROM THE NAVGEM AND GFS GLOBAL MODELS. THE TIMING OF THE TURN TO THE NORTH AND EVENTUAL RECURVE TOWARD THE NORTHEAST WILL DEPEND ON THE STRENGTH OF THE MID-LATITUDE TROUGH THAT IS FORECAST TO ERODE THE WESTERN PORTION OF THE STR. VWS IS FORECAST TO REMAIN LOW AND ACCOMPANIED BY FAVORABLE OUTFLOW THROUGH MOST OF THE FORECAST PERIOD, THEREFORE CONTINUED INTENSIFICATION IS FORECAST, WITH A PEAK INTENSITY OF 95 KNOTS AT TAU 96. DUE TO THE MODEL UNCERTAINTY BEYOND TAU 72, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST TRACK DURING THIS PORTION OF THE FORECAST.// NNNN NNNN