WDPN31 PGTW 080300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (CONSON) WARNING NR 009// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 13.6N 121.3E INITIAL INTENSITY: 55 KTS GEOGRAPHIC REFERENCE: 62 NM SOUTH-SOUTHEAST OF MANILA, PHILIPPINES MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 07 KTS SIGNIFICANT WAVE HEIGHT: 14 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TS 18W HAS MAINTAINED INTENSITY WITH NO SIGNIFICANT DEGRADATION OVER THE PAST SIX HOURS. IN FACT, A 072216Z SSMIS 91GHZ MICROWAVE IMAGE INDICATES A SLIGHT IMPROVEMENT IN CONVECTIVE STRUCTURE WITH A COMPACT CORE SURROUNDING A DEFINED MICROWAVE EYE FEATURE, WHICH SUPPORTS THE INITIAL POSITION WITH MEDIUM CONFIDENCE. HOWEVER, OVER THE PAST TWO HOURS ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS WEAKENING DEEP CONVECTION AS THE SYSTEM TRACKS OVER SOUTHERN LUZON. THE INITIAL INTENSITY OF 55 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON AN AVERAGE OF RJTD, RCTP AND KNES DVORAK ESTIMATES, CONSISTENT WITH THE MICROWAVE EYE FEATURE. UPPER-LEVEL ANALYSIS INDICATES FAVORABLE OUTFLOW CONDITIONS WITH LOW VERTICAL WIND SHEAR. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTH. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: NO CURRENT ESTIMATE RJTD: T3.0 - 45 KTS RCTP: T3.0 - 45 KTS KNES: T4.0 - 65 KTS CIMSS ADT: 72 KTS AT 072340Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 5-10 KTS SST: 27-28 CELSIUS OUTFLOW: MODERATE RADIAL OTHER FACTORS: INTERACTION WITH LAND ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM 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 WEAKEN SLIGHTLY TO 50 KNOTS AS IT TRACKS NORTHWESTWARD OVER SOUTHERN LUZON. AFTER TAU 24, THE SYSTEM WILL GRADUALLY STRENGTHEN OVER THE SOUTH CHINA SEA AS IT TRACKS WESTWARD TO WEST-NORTHWESTWARD TOWARD HAINAN ISLAND. THERE IS A LOW PROBABILITY THAT WEAK INTERACTION AND POSSIBLY MINOR TRACK CHANGES MAY OCCUR AFTER TAU 48 AS TS 18W CLOSES WITHIN ABOUT 485NM OF TY 19W AT TAU 60. MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, WHICH IS INDICATING BINARY INTERACTION WITH TY 19W AND TRACKING QUICKLY POLEWARD AND MERGING, NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT THROUGH TAU 72. THEREFORE, THERE IS MEDIUM CONFIDENCE IN THE JTWC FORECAST TRACK THROUGH TAU 72. AFTER TAU 72, THE NUMERICAL MODELS DIVERGE WITH A LARGE SPREAD (265NM AT TAU 120) IN SOLUTIONS RANGING FROM A WESTWARD TRACK TOWARD DA NANG TO A NORTHWESTWARD TRACK OVER HAINAN ISLAND LENDING MEDIUM CONFIDENCE TO THE JTWC FORECAST TRACK IN THE EXTENDED PERIOD. THERE IS ALSO SIGNIFICANT UNCERTAINTY IN THE JTWC INTENSITY FORECAST IN THE EARLY PERIOD DUE TO LAND INTERACTION, AND IN THE LATER PERIOD DUE TO INCREASING VERTICAL WIND SHEAR. AS A SIDE NOTE, THE 061200Z BEST TRACK POSITION WAS REANALYZED TO TYPHOON STRENGTH (65 KNOTS) BASED ON SURFACE OBSERVATIONS FROM GUIUAN, PHILIPPINES. THE 061300Z REPORT INDICATED WEST-SOUTHWESTERLY WINDS AT 62 KNOTS AT AN ELEVATION OF 60M. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN