WDPN31 PGTW 070900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (CONSON) WARNING NR 006// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 12.5N 123.3E INITIAL INTENSITY: 45 KTS GEOGRAPHIC REFERENCE: 184 NM SOUTHEAST OF MANILA, PHILIPPINES MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 11 KTS SIGNIFICANT WAVE HEIGHT: 18 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL STORM 18W CONTINUES TO WEAKEN AS IT HAS TRACKED ACROSS MASBATE ISLAND OVER THE PREVIOUS SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) STILL INDICATES THAT THE OVERALL ROTATION ASSOCIATED WITH TS CONSON REMAINS INTACT, BUT STRUCTURALLY IT HAS BECOME SOMEWHAT RAGGED. A 070527Z AMSR2 89GHZ MICROWAVE IMAGE REVEALS WEAK SPIRAL BANDS WRAPPING INTO A RAGGED AND BROAD LOW EMISSIVITY REGION. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AMSR2 IMAGE AND SUPPORTED BY ANIMATED RADAR DATA FROM THE PHILIPPINES RADAR NETWORK. SURFACE OBSERVATIONS FROM MASBATE CITY INDICATE SOUTHEASTERLY WINDS WITH GUSTS UP TO NEAR 40 KNOTS AND SEA LEVEL PRESSURES NEAR 997MB. THE INITIAL INTENSITY OF 45 KNOTS IS BASED ON A PGTW DATA-T OF T3.0 (45 KTS) AND AN RJTD CI OF T3.0, WITH SUPPORT FROM THE SURFACE OBSERVATION FROM MASBATE CITY. ANIMATED WATER VAPOR IMAGERY SHOWS THE SYSTEM HAS GOOD RADIAL OUTFLOW WITH WEAK POLEWARD FLOW INTO A TUTT CELL POSITIONED TO THE NORTHEAST. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: NEAR EQUATORIAL RIDGE (NER) CENTERED NEAR PALAU AND DEEP SUBTROPICAL RIDGE (STR) EXTENDING FROM SOUTHERN CHINA TO EAST OF OKINAWA. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.5 - 55 KTS RJTD: T3.0 - 45 KTS CIMSS ADT: 53 KTS AT 070540Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 0-5 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TS 18W IS FORECAST TO CONTINUE TRACKING NORTHWESTWARD, GENERALLY TOWARDS THE MANILA METRO AREA, OVER THE NEXT 24 HOURS AND EMERGE INTO THE SOUTH CHINA SEA ONCE MORE NEAR TAU 36, MOVING ALONG THE WESTERN PERIPHERY OF THE NER CENTERED TO THE EAST AND THE SOUTHERN PERIPHERY OF THE STR TO THE NORTH. AFTER EMERGING INTO THE SOUTH CHINA SEA, THE TRACK OF 18W IS FORECAST TO FLATTEN OUT, BECOMING MORE WEST-NORTHWESTWARD AS THE STR TO THE NORTH BECOMES THE DOMINATE STEERING MECHANISM. AS THE SYSTEM TRACKS OVER THE SIBUYAN SEA IT IS LIKELY TO MAINTAIN ITS CURRENT INTENSITY, AND COULD POTENTIALLY SEE SHORT-TERM INTENSIFICATION WHILE OVER THE WARM WATERS, BEFORE MAKING ANOTHER LANDFALL ON THE SOUTHERN COAST OF LUZON SOUTHEAST OF MANILA. THE RUGGED TERRAIN WILL SERVE TO WEAKEN TS 18W TO 40 KNOTS BEFORE REEMERGING OVER OPEN WATERS NEAR TAU 36. ENVIRONMENTAL CONDITIONS OF LOW (10-15 KTS) VWS, MODERATE WESTWARD OUTFLOW AND WARM (29C) SSTS, ARE EXPECTED TO REMAIN FAVORABLE AND SUPPORT SLOW BUT STEADY INTENSIFICATION AFTER TAU 48, REACHING A PEAK OF 75 KNOTS BY TAU 120. MODEL DISCUSSION: TRACK GUIDANCE REMAINS IN GOOD AGREEMENT THROUGH TAU 48, WITH A SPREAD IN THE GUIDANCE ENVELOPE OF ONLY 75NM AT TAU 48. HOWEVER, THE GUIDANCE IS SHOWING INCREASING UNCERTAINTY FROM TAU 72 ONWARDS, WITH MULTIPLE MODELS (GFS, GFS ENSEMBLE, HWRF) NOW SHOWING A SIGNIFICANT SLOWDOWN IN THE MID-SOUTH CHINA SEA AROUND TAU 72. GLOBAL MODEL FIELDS FOR THE GFS SHOW THE STR TO THE NORTH WEAKENING AND MOVING INTO A POSITION DUE NORTH OF TS 18W, SLACKENING THE STEERING FLOW AND RESULTING IN A VERY SLOW OR EVEN QUASI-STATIONARY MOVEMENT AFTER TAU 96. THE NAVGEM MODEL MEANWHILE SHOWS THE SYSTEM SIGNIFICANTLY WEAKER, AND BEING ABSORBED INTO THE OVERALL FLOW ASSOCIATED WITH TY 19W TO THE NORTH. HOWEVER, THIS SCEANRIO IS CONSIDERED HIGHLY UNLIKELY AND IS DISCOUNTED. THE REMAINDER OF THE GUIDANCE REMAINS CONSISTENT WITH PREVIOUS RUNS AND SUPPORTS THE JTWC FORECAST TRACK WHICH LIES ON THE SOUTHERN SIDE OF THE CONSENSUS ENVELOPE WITH MEDIUM CONFIDENCE. THERE IS MEDIUM CONFIDENCE IN THE INTENSITY FORECAST THROUGH TAU 72 DUE TO THE NEAR-TERM UNCERTAINTIES SURROUNDING THE EFFECTS OF LAND INTERACTION. IN THE LONG-TERM, THERE IS LOW CONFIDENCE, AS THE ULTIMATE PEAK INTENSITY OF TS 18W IS HIGHLY DEPENDENT ON HOW MUCH OF THE SYSTEM REMAINS AFTER TRANSITING ACROSS LUZON. GUIDANCE IS IN FAIRLY GOOD AGREEMENT, THOUGH THE HWRF CONTINUES TO SHOW UNREALISTIC RAPID INTENSIFICATION IN THE FIRST 12 HOURS WHILE THE SYSTEM IS OVER THE SIBUYAN SEA. THE JTWC FORECAST LIES CLOSE TO THE INTENSITY CONSENSUS THROUGH THE DURATION OF THE FORECAST PERIOD. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN