MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (CHABA) WARNING NR 05// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION 21W (CHABA), LOCATED APPROXIMATELY 137 NM WEST-NORTHWEST OF ANDERSEN AFB, HAS TRACKED WESTWARD AT 13 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS FLARING CONVECTION BRIEFLY PROPAGATING TOWARD THE SOUTHWEST IN THE LAST SEVERAL HOURS, WITH 282113Z AND 282201Z SSMIS 91GHZ MICROWAVE IMAGERY SUGGESTING THAT THE LOW-LEVEL CIRCULATION CENTER (LLCC) IS LOCATED NEAR THE NORTHERN EDGE OF THE DEEPEST CONVECTION. THIS IS CORROBORATED BY THE LOW-LEVEL CLOUD LINES EVIDENT IN ANIMATED VISIBLE SATELLITE IMAGERY. ONLY A BROAD COUNTERCLOCKWISE CIRCULATION IS EVIDENT IN THE GUAM RADAR REFLECTIVITY, BUT RADAR BEAM HEIGHTS ARE INCREASING AS TD CHABA MOVES AWAY FROM THE RADAR SITE. THE CURRENT POSITION IS PLACED WITH LOW CONFIDENCE BY AID OF THE AFOREMENTIONED IMAGERY. DVORAK INTENSITY ESTIMATES HAVE FINALLY COME DOWN SINCE THE LAST SEVERAL WARNING CYCLONES, RANGING FROM T2.0 (KNES) TO T2.5 (PGTW AND RJTD), WHICH BETTER SUPPORTS THE WIND VECTORS OBSERVED IN SCATTEROMETER DATA. A PARTIAL 282029Z WINDSAT PASS DEPICTS 20 TO 25 KNOT WINDS PREDOMINANTLY LOCATED ON THE NORTHERN SEMICIRCLE OF TD CHABA. BASED ON A BLEND OF THE ABOVE DATA, THE CURRENT INTENSITY IS MAINTAINED AT 30 KNOTS. TD CHABA CONTINUES TO BE LOCATED IN A MARGINAL ENVIRONMENT WITH WARM SSTS AND LOW VERTICAL WIND SHEAR, BUT UNDER RESTRICTED POLEWARD OUTFLOW DUE TO A WEAK MID-LEVEL TUTT CELL PASS TO ITS NORTH. TD CHABA IS TRACKING WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE OF A SYNOPTIC SCALE, 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 CONTINUE TO STEER TD CHABA TOWARD THE WEST- NORTHWEST WITH A GRADUAL TURN TOWARD THE NORTHWEST. SLOW INTENSIFICATION IS FORECAST IN THE NEAR TERM, FOLLOWED BY MORE STEADY INTENSIFICATION BEYOND TAU 24 WHEN THE TUTT CELL CURRENTLY TO THE NORTH OF TD CHABA WEAKENS, SLIDES WESTWARD, AND OPENS UP A POLEWARD OUTFLOW CHANNEL. WITH CONTINUED LOW VERTICAL WIND SHEAR AND IMPROVED OUTFLOW, TD CHABA IS FORECAST TO REACH TYPHOON INTENSITY IN THE 48 TO 72 HOUR TIME FRAME. DYNAMIC MODEL GUIDANCE REMAINS IN GOOD AGREEMENT WITH A GENERAL WEST-NORTHWEST TO NORTHWESTWARD TRACK THROUGH TAU 72, AND THE OFFICIAL JTWC FORECAST TRACK REMAINS NEAR THE MULTIMODEL CONSENSUS. C. IN THE EXTENDED PERIOD, DYNAMIC MODEL GUIDANCE DIVERGES GREATLY BEYOND TAU 72, WITH THE GFS AND GFS/JGSM ENSEMBLES REMAINING ON THE RIGHT SIDE OF THE MODEL GUIDANCE ENVELOPE. THE OFFICIAL FORECAST TRACK REMAINS TO THE WEST OF THE MULTIMODEL CONSENSUS AT TAU 96 AND 120, BUT THESE LATER POSITIONS MAY NEED TO BE SHIFTED EASTWARD IN FUTURE WARNINGS IF THE MODEL SUITE FOLLOWS THE EASTWARD SHIFT THAT WAS EXHIBITED BY ECMWF EARLIER TODAY. 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. VERTICAL WIND SHEAR IS FORECAST TO REMAIN LOW AND ACCOMPANIED BY FAVORABLE OUTFLOW THROUGH MOST OF THE FORECAST PERIOD, SO CONTINUED INTENSIFICATION IS FORECAST. HOWEVER, A TRACK TO THE WEST OF OKINAWA WOULD TAKE TD CHABA OVER COOLER SSTS, WHICH MAY HINDER INTENSIFICATION AT THE LATER TAUS. THE OFFICIAL INTENSITY FORECAST LIES CLOSE TO THE INTENSITY CONSENSUS, BUT THE ACTUAL INTENSIFICATION RATE OF TD CHABA WILL STRONGLY DEPEND ON ITS EVENTUAL TRACK. THIS UNCERTAINTY LEADS TO A LOW CONFIDENCE IN BOTH THE JTWC TRACK AND INTENSITY FORECASTS BEYOND TAU 72.// NNNN NNNN