WDPN31 PGTW 010900 SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHOI-WAN) WARNING NR 012// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 04W (CHOI-WAN), LOCATED APPROXIMATELY 322 NM EAST-SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED NORTHWESTWARD AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A SMALL, EXPOSED AND INCREASINGLY DISORGANIZED LOW-LEVEL CIRCULATION (LLC) WITH A RECENT FLARE UP OF DEEP CONVECTION TO THE SOUTHWEST OF THE CENTER ALONG THE COAST OF SAMAR. EIR SUBSEQUENT TO THE 011200Z HOUR SHOWS THE CONVECTIVE MASS EXPANDING, WITH THE CIRRUS SHIELD BEGINNING TO EXPAND OVER THE ASSESSED CENTER. THE INITIAL POSITION IS PLACED WITH MODERATE CONFIDENCE BASED ON THE EXPOSED CIRCULATION CENTER TO THE EAST OF CENTRAL SAMAR. THE INITIAL INTENSITY HAS BEEN LOWERED TO 30 KNOTS WITH HIGH CONFIDENCE, IN LINE WITH MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES RANGING BETWEEN T1.5 (25KTS) AND T2.5 (35 KTS) FROM PGTW AND RJTD, AND A RAW ADT ESTIMATE OF T1.8. A RECENT (010959Z) SCATTEROMETER PASS AND A 011100Z SURFACE PRESSURE READING OF 999.5MB FROM BORONGAN, JUST SOUTHWEST OF THE CENTER, LENT ADDITIONAL CONFIDENCE TO THE 30KT INITIAL INTENSITY. ENVIRONMENTAL ANALYSIS REVEALS CONDITIONS INIMICAL TO DEVELOPMENT, WITH MODERATE (15-20 KTS) VWS, CONVERGENT FLOW AND MID-LEVEL DRY AIR ASSOCIATED WITH TUTT CELL LOCATED JUST NORTHEAST OF THE LLCC, OFFSETTING THE GOOD EQUATORWARD OUTFLOW AND WARM (30C) SSTS. TD 04W IS TRACKING NORTHWEST ALONG THE WESTERN PERIPHERY OF THE DEEP STR TO THE NORTHEAST. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE OVERALL FORECAST PHILOSOPHY, HOWEVER IN LIGHT OF THE RECENT WEAKENING TREND AND IMMINENT PASSAGE OVER RUGGED TERRAIN, THE FORECAST HAS BEEN TRUNCATED TO 72 HOURS AND THE SYSTEM IS NOW EXPECTED TO DISSIPATE VICE UNDERGO ETT. B. OVER THE NEXT 36 HOURS, TD 04W IS FORECAST TO CONTINUE TRACKING NORTHWESTWARD, MOVING OVER NORTHERN SAMAR, SOUTHEASTERN LUZON, AND THE PALILLO ISLANDS, BEFORE MAKING A FINAL LANDFALL ALONG THE EASTERN COAST OF NORTHERN LUZON JUST PRIOR TO TAU 36. THE PREVIOUSLY MENTIONED TUTT CELL CONTINUING TO TRACK WESTWARD AND WILL MOVE OVER TOP OF TD 04W WITHIN THE NEXT 12 HOURS AND WILL CONTINUE TO SQUASH ANY SIGNIFICANT INTENSIFICATION. BY TAU 24 THE TUTT IS EXPECTED TO DISSIPATE TO THE WEST, AND AS THE SYSTEM MOVES OVER WATER ONCE MORE, THE COMBINATION OF IMPROVED OUTFLOW SHOULD ALLOW TD 04W TO MAINTAIN AN INTENSITY OF 30 KTS AS IT CONTINUES TRACKING NORTHWESTWARD. ONCE OVER THE RUGGED TERRAIN OF NORTHERN LUZON HOWEVER, THE RUGGED TERRAIN WILL SERVE TO DISRUPT THE LOW LEVEL CIRCULATION (LLC), LEADING TO A SLOW WEAKENING TREND. WHAT REMAINS OF THE LLC WILL EMERGE INTO THE LUZON STRAIT AROUND TAU 48 AND TRACK NORTHEASTWARD INTO A BREAK IN THE STEERING RIDGE. HOWEVER, INCREASING VWS AND CONVERGENT FLOW ALOFT WILL CONSPIRE TO DISSIPATE THE SYSTEM NO LATER THAN TAU 72, AS IT MERGES WITH THE SOUTHERN END OF A BAROCLINIC ZONE EAST OF TAIWAN. DISSIPATION OVER NORTHERN LUZON, PRIOR TO REEMERGENCE OVER WATER, ALSO REMAINS A POSSIBILITY. NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT ON THE OVERALL TRACK THROUGH TAU 72, THOUGH THE GFS HAS BEGUN TO TREND MORE WESTWARD INTO THE SOUTH CHINA SEA AFTER TAU 36. THE JTWC FORECAST TRACK LIES CLOSE TO THE MULTI-MODEL CONSENSUS MEAN THROUGH THE DURATION OF THE FORECAST PERIOD, BUT DUE TO THE UNCERTAINTY ASSOCIATED WITH THE IMPACT OF THE LAND INTERACTION AND THE DOWNSTREAM EFFECTS OF THAT INTERACTION ON BOTH TRACK AND INTENSITY, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST.// NNNN