MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (CHABA) WARNING NR 04// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 21W (CHABA), LOCATED APPROXIMATELY 67 NM NORTHWEST OF ANDERSEN AFB, HAS TRACKED WESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS FLARING CONVECTION ASSOCIATED WITH TD CHABA, WITH A 281704Z AMSU 89GHZ MICROWAVE IMAGE SHOWING THE DEEPEST CONVECTIVE CELLS DISPLACED TO THE SOUTHEAST OF THE LOW-LEVEL CIRCULATION CENTER (LLCC). THIS ASYMMETRIC STRUCTURE IS ALSO EVIDENT IN THE RADAR REFLECTIVITY FROM ANDERSEN AFB. THE CURRENT POSITION IS PLACED WITH FAIR CONFIDENCE BY AID OF THE AFOREMENTIONED IMAGERY. DVORAK INTENSITY ESTIMATES RANGE FROM T2.5 TO T3.0 FROM ALL AGENCIES, WHICH APPEARS TO BE TOO HIGH BASED ON OTHER OBSERVATIONAL DATA. THE CURRENT INTENSITY IS MAINTAINED AT 30 KNOTS BASED ON THE EARLIER 281128Z PARTIAL ASCAT PASS AND THE STEADY-STATE NATURE OF THE FLARING CONVECTION SINCE THAT TIME. TD CHABA IS LOCATED IN A MARGINAL ENVIRONMENT WITH WARM SSTS AND LOW VERTICAL WIND SHEAR, BUT UNDER RESTRICTED POLEWARD OUTFLOW DUE TO A TUTT CELL TO THE NORTHEAST. TD CHABA IS TRACKING WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE 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 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 NORTHEAST OF TD CHABA SLIDES WESTWARD AND OPENS UP A POLEWARD OUTFLOW CHANNEL IN ITS WAKE. 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 IS PLACED NEAR THE MULTIMODEL CONSENSUS. C. IN THE EXTENDED PERIOD, DYNAMIC MODEL GUIDANCE DIVERGES GREATLY BEYOND TAU 72, WITH THE GFS AND GFS/JGSM ENSEMBLES AT THE RIGHT SIDE OF THE MODEL GUIDANCE ENVELOPE. SINCE THE LAST WARNING, THE EGRR HAS SHIFTED TO THE WEST OF OKINAWA, WHEREAS THE ECMWF HAS SHIFTED TO THE EAST. DIFFERENCES IN MODEL SOLUTIONS ARE DUE TO DIFFERENCES IN THE FORECAST INTENSITY OF THE STR. THE OFFICIAL JTWC FORECAST TRACK REMAINS CLOSE TO THE PREVIOUS WARNING, STAYING TO THE LEFT OF THE MULTIMODEL CONSENSUS BEYOND TAU 72. HOWEVER, THIS MAY NEED TO BE ADJUSTED DEPENDING ON THE EVOLUTION OF FUTURE MODEL GUIDANCE. VERTICAL WIND SHEAR IS FORECAST TO REMAIN LOW WITH GOOD OUTFLOW, BUT THE PRESENCE OF COOLER SSTS IN THE REGION BETWEEN OKINAWA AND TAIWAN WILL COMPLICATE THE LONG-RANGE INTENSITY FORECAST. THE OFFICIAL INTENSITY FORECAST LIES CLOSE TO THE INTENSITY CONSENSUS, BUT THIS IS A CASE WHERE 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