MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 001// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 21W (TWENTYONE), LOCATED APPROXIMATELY 542 NM EAST- NORTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WEST-NORTHWESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS PERSISTENT DEEP CONVECTION FLARING OVER A SLOWLY-DEVELOPING LOW-LEVEL CIRCULATION (LLC). A 160840Z SSMIS 91GHZ IMAGE DEPICTS A BROAD, DEFINED LLC WITH FRAGMENTED DEEP CONVECTIVE BANDING OVER THE NORTHEAST QUADRANT AND A WEAKLY-DEFINED CENTER. A 161204Z ASCAT-C IMAGE SHOWS A DEFINED, ELONGATED CIRCULATION WITH WESTERLY WINDS OVER THE SOUTHERN SEMICIRCLE AND 20-25 KNOT WINDS OVER THE EASTERN SEMICIRCLE, WHICH SUPPORTS THE CURRENT INTENSITY ASSESSMENT OF 25 KNOTS. THIS ASCAT IMAGE ALSO SUPPORTS THE CURRENT POSITION WITH GOOD CONFIDENCE. TD 21W IS TRACKING ALONG THE SOUTHERN PERIPHERY OF THE LOW-LEVEL SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTH. 3. FORECAST REASONING. A. THIS IS THE INITIAL PROGNOSTIC REASONING MESSAGE AND SETS THE FORECAST PHILOSOPHY ON THIS SYSTEM. B. TD 21W WILL TRACK WEST-NORTHWESTWARD THEN WESTWARD UNDER THE STEERING INFLUENCE OF THE LOW-LEVEL STR THROUGH TAU 36. AFTER TAU 36, THE SYSTEM IS EXPECTED TO TURN WEST-SOUTHWESTWARD AS IT BEGINS TO INTERACT WITH A NORTHEAST SURGE EVENT. DUE TO THE WEAK NATURE OF THE SYSTEM, MODEL GUIDANCE IS LIMITED TO AEMN, NVGM AND AVNO, HOWEVER, THESE MODELS ARE IN GOOD AGREEMENT. THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK. THE ECMWF MODEL TRACKER ALSO SUPPORTS THE JTWC FORECAST TRACK INTO NORTHERN LUZON BUT REFLECTS A GENERALLY WEAK SYSTEM. DUE TO THE MARGINAL ENVIRONMENT, TD 21W IS FORECAST TO INTENSIFY SLIGHTLY TO 30 KNOTS THEN QUICKLY DISSIPATE AS IT INTERACTS WITH THE NORTHEAST SURGE AND TRACKS OVER LUZON.// NNNN NNNN