WDPN32 PGTW 230300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN) WARNING NR 005// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 18.1N 132.6E INITIAL INTENSITY: 40 KTS GEOGRAPHIC REFERENCE: 607 NM NORTH-NORTHWEST OF YAP MOVEMENT PAST 6 HOURS: WESTWARD AT 08 KTS SIGNIFICANT WAVE HEIGHT: 15 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN UPPER LEVEL CONVECTIVE STRUCTURE THAT APPEARS TO BE SLIGHTLY TILTED AND DISPLACED FROM THE NOW PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). AN EARLIER 222039Z SSMIS MICROWAVE IMAGE ALSO CONFIRMS THE SLIGHTLY TILTED STRUCTURE OF THE SYSTEM, AS THE HIGHEST BRIGHTNESS TEMPERATURES ARE LOCATED WITHIN THE SOUTHWESTERN QUADRANT IN RELATION TO THE LOW LEVEL CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC, AND PLACED CLOSE TO AGENCY DVORAK FIXES. THE INITIAL INTENSITY IS PLACED WITH LOW CONFIDENCE BASED ON A COMBINATION OF SUBJECTIVE AND OBJECTIVE AGENCY DVORAK INTENSITY ESTIMATES, PGTW T2.5, KNES T3.0, RJTD 2.0, ALONG WITH ADT 34 KTS, AIDT 40 KTS, AND SATCON 40 KTS . INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE NORTHEAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T2.0 - 30 KTS KNES: T3.0 - 45 KTS CIMSS SATCON: 40 KTS AT 222125Z CIMSS ADT: 34 KTS AT 222340Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 10-15 KTS SST: 29-30 CELSIUS OUTFLOW: WEAK WESTWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: LOW INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TS 18W HAS BECOME PARTIALLY EXPOSED OVER THE LAST 6 HOURS AS THE SYSTEM IS CURRENTLY SITUATED IN A MARGINALLY FAVORABLE ENVIRONMENT, CHARACTERIZED BY WARM SEA SURFACE TEMPERATURES (SSTS) (29-30C), WEAK RADIAL OUTFLOW, OFFSET BY MODERATE VERTICAL WIND SHEAR (10-15 KNOTS) AND MID LEVEL DRY AIR SURROUNDING THE SYSTEM. OVER THE NEXT 24 HOURS, THE SYSTEM IS FORECAST TO TRACK GENERALLY WESTWARD UNDER THE INFLUENCE OF A DEEP-LAYER SUBTROPICAL RIDGE TO THE NORTHEAST WHICH HAS TAKEN OVER AS THE DOMINATE STEERING FEATURE AND WILL CONTINUE TO STEER 18W TOWARD THE COAST OF LUZON, BEFORE MAKING LANDFALL SHORTLY AFTER TAU 48. RAPID INTENSIFICATION GUIDANCE IS TRIGGERED, HOWEVER BECAUSE THE SYSTEM WILL HAVE EASTERLY CONVERGENT FLOW ALOFT WITH OVERALL WEAK UPPER-LEVEL OUTFLOW, AS WELL AS MID LEVEL DRY AIR TO CONTEND WITH, THE FORECAST WILL MAINTAIN A GRADUAL INTENSIFICATION UP TO 65 KNOTS PRIOR TO LANDFALL INTO LUZON. THIS LAND INTERACTION WILL LIKELY PUT A CAP ON INTENSIFICATION AND POSSIBLY DECREASE INTENSITY, BUT ONLY SLIGHTLY GIVEN THE SHORT DURATION OVER LAND. HOWEVER, BY TAU 72, THE SYSTEM MOVES BACK OVER THE WARM WATERS OF THE SOUTH CHINA SEA, AS THE UPPER LEVEL ENVIRONMENT BECOMES MORE FAVORABLE FOR INTENSIFICATION, CHARACTERIZED BY ENHANCED OUTFLOW AND LOW VERTICAL WIND SHEAR. THUS, GIVEN THE FAVORABLE ENVIRONMENTAL FACTORS, THE OFFICIAL JTWC FORECAST SHOWS AN INCREASE UP TO 80 KNOTS BY TAU 96. BY TAU 120, 18W WILL CONTINUE A STRAIGHT RUNNING WESTWARD TRACK BEFORE EVENTUALLY MAKING LANDFALL INTO NORTHERN VIETNAM. MODEL DISCUSSION: NUMERICAL MODELS ARE IN OVERALL TIGHT AGREEMENT WITH THE EXCEPTION OF JGSM WHICH MAKES UP THE SOUTHERN OUTLIER, WHILE NAVGEM IS THE NORTHERN OUTLIER. OVER THE NEXT 48 HOURS, TRACK SPREAD WILL GRADUALLY INCREASE TO 50 NM. AFTER THE SYSTEM MOVES PAST LUZON AND BEYOND TAU 72, GUIDANCE BEGINS TO DIVERGE EVEN FURTHER TO A 100 NM CROSS-TRACK SPREAD, HOWEVER GUIDANCE REMAINS IN OVERALL GOOD AGREEMENT REGARDING ALONG-TRACK SPREAD. INTENSITY GUIDANCE INDICATES UNCERTAINTY DUE TO RAPID INTENSIFICATION GUIDANCE BEING TRIGGERED THROUGH TAU 48, HOWEVER INTENSITY GUIDANCE BECOMES MORE IN LINE FROM TAU 72 THROUGH TAU 96. THUS, THE JTWC INTENSITY FORECAST IS PLACED WITH MEDIUM CONFIDENCE OVERALL. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN