WDPN31 PGTW 121500 MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 23W (BANYAN) WARNING NR 11// RMKS/ 1. FOR METEOROLOGISTS. 2. 12 HOUR SUMMARY AND ANALYSIS. A. TROPICAL DEPRESSION (TD) 23W (BANYAN), LOCATED APPROXIMATELY 240 NM SOUTH OF MANILA, PHILIPPINES, HAS TRACKED WEST-NORTHWESTWARD AT 16 KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY SHOWS A BROAD SYSTEM WITH AN EXPANSIVE AREA OF CONVECTION THAT EXTENDS OVER THE NORTHERN VISAYAS AND AS FAR NORTH AS MANILA BAY. DEEP CONVECTION IS LIMITED PREDOMINATELY TO THE NORTHEAST QUADRANT. HOWEVER, THE WIDE-SPREAD CONVECTION AND LOW INTENSITY OF THE SYSTEM IS MAKING FOR LOWER THAN AVERAGE POSITION CONFIDENCE. THE CURRENT POSITION IS BASED ON INFRARED ANIMATION AND EXTRAPOLATED FROM A HIGHER CONFIDENCE FIX AT 120600Z AND IS SUPPORTED BY A 121040Z TRMM IMAGE. THE LATEST AMSU TEMPERATURE CROSS SECTION REVEALS THAT THE MID-LEVEL WARM ANOMALY HAS DISAPPEARED. ANIMATED WATER VAPOR IMAGERY INDICATES MILD IMPINGEMENT ON OUTFLOW OVER THE NORTHERN SEMICIRCLE AND WELL-DEVELOPED EQUATORWARD OUTFLOW. THE 120000Z PGTW UPPER LEVEL STREAMLINE ANALYSIS REVEALS SOME DIFFLUENCE IN THE EASTERLY WIND FIELD ABOVE THE SYSTEM, BUT OVERALL THE UPPER LEVEL ENVIRONMENT IS POOR, WITH 20 TO 30 KNOTS OF VERTICAL WIND SHEAR ABOVE THE LLCC. THE EASTERLIES ARE ASSOCIATED WITH A STATIONARY UPPER LEVEL ANTICYCLONE CENTERED NEAR 19N 117E. THE STEERING FORCE FOR TD 23W IS A LOW TO MID-LEVEL STEERING RIDGE CENTERED JUST EAST OF TAIWAN. THE RIDGE IS RE-ORIENTING EASTWARD AS A DEEP MID-LATITUDE TROUGH DIGS OVER SOUTHEASTERN CHINA. 3. A. THE FORECAST PHILOSOPHY HAS NOT CHANGED. B. TD 23W WILL TRACK ALONG THE SOUTHWESTERN PERIPHERY OF THE LOW- TO MID-LEVEL STEERING RIDGE, AND AS THE RIDGE GRADUALLY SHIFTS AWAY FROM TAIWAN, TD 23W WILL STEADILY VEER TO THE NORTHWEST. SEA SURFACE TEMPERATURES ALONG TRACK WILL FALL TO 29 TO 30 DEGREES WEST OF PALAWAN WHILE VERTICAL WIND SHEAR WILL REMAIN ABOVE 20 KNOTS. THEREFORE TD 23W WILL INTENSIFY VERY SLOWLY AS IT TRACKS THROUGH THE SOUTH CHINA SEA. GUIDANCE IS COMING INTO MUCH BETTER AGREEMENT REGARDING BOTH TRACK AND INTENSITY THROUGH THE INITIAL 72 HOURS AND POINTS TO A VERY SLOW RATE OF INTENSIFICATION WITH A PEAK INTENSITY WELL BELOW TYPHOON STRENGTH. BETWEEN TAU 48 AND TAU 72, TD 23W WILL LEAVE THE INFLUENCE OF THE OCEANIC ANTICYCLONE AND BEGIN TO INTERACT WITH A BUILDING ANTICYCLONE OVER SOUTHEASTERN CHINA. C. THE ANTICYCLONE BUILDING ALONG THE SOUTHERN CHINESE COAST WILL BECOME THE DOMINANT STEERING MECHANISM NEAR TAU 72, DRIVING THE SYSTEM WESTWARD TOWARDS VIETNAM. ALTHOUGH VERTICAL WIND SHEAR WILL EASE SLIGHTLY, SEA SURFACE TEMPERATURES WILL ALSO FALL, AND TD 23W WILL BEGIN WEAKENING WELL BEFORE LANDFALL. RECENT EXTENDED RANGE MODEL GUIDANCE HAS ALSO COME INTO MUCH BETTER AGREEMENT.// NNNN