WDPN31 PGTW 211500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 25W (TWENTYFIVE) WARNING NR 008// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 20.4N 121.7E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 350 NM NORTH OF MANILA, PHILIPPINES MOVEMENT PAST 6 HOURS: WESTWARD AT 13 KTS SIGNIFICANT WAVE HEIGHT: 15 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: OVER THE PAST SIX HOURS, THE CONVECTIVE AND OVERALL STRUCTURE OF TROPICAL DEPRESSION (TD) 25W HAS ERODED SIGNIFICANTLY. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS THIN UPPER-LEVEL CIRRUS, REMNANTS OF A NOW DISSIPATED CENTRAL DENSE OVERCAST (CDO), OBSCURING THE LOW LEVEL CIRCULATION CENTER (LLCC) AND ISOLATED, DISORGANIZED CONVECTION FLARING UP AT VARIOUS POINTS AROUND THE CIRCULATION. THE EIR ALSO POINTS TO A MID-LEVEL VORTEX DISPLACED TO THE SOUTHWEST WHICH DRAWS ATTENTION AS A CANDIDATE POSITION OF THE LLCC, BUT THIS IS NOT THE CIRCULATION WE ARE LOOKING FOR. A 210912Z GMI 37GHZ MICROWAVE IMAGE REVEALED THE LOW-LEVEL STRUCTURE, WITH SHALLOW BANDING FEATURES SPIRALING INTO A WEAK LLCC ON THE EASTERN EDGE OF THE CDO THAT WAS PRESENT AT THE TIME. ADDITIONALLY, A SHIP OBSERVATION AT 20.1N 121.2E REPORTED NORTH-NORTHEASTERLY WINDS, WHILE CALAYAN ISLAND REPORTED WESTERLY WINDS, CONSTRAINING THE INITIAL POSITION TO THE NORTH AND EAST OF THESE POSITIONS, WITH HIGH CONFIDENCE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, ON THE HIGHER END OF THE AGENCY FIX INTENSITIES AND JUST SLIGHTLY BELOW THE OBJECTIVE ADT ESTIMATE. ENVIRONMENTAL CONDITIONS REMAIN MARGINALLY FAVORABLE WITH LOW VWS, AND WARM SSTS BEING OFFSET BY A LACK OF LOW-LEVEL ORGANIZATION AND WEAK RADIAL OUTFLOW. ADDITIONALLY, SOUNDINGS FROM SHANTOU, CHINA SHOW A VERY STRONG INVERSION AT ABOUT 4000 FEET, WITH VERY DRY AIR EXTENDING UP TO ABOUT 8000 FEET. THIS DRY AND STABLE AIR IS BEING ADVECTED SOUTHWARD INTO THE WESTERN SIDE OF THE TD 25W, FURTHER INHIBITING CONVECTIVE DEVELOPMENT AND LOW-LEVEL CONSOLIDATION. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER STR COMPLEX WITH CENTERS TO THE NORTHEAST AND NORTHWEST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.0 - 25 KTS KNES: T2.0 - 30 KTS CIMSS ADT: 33 KTS AT 211140Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 0-5 KTS SST: 27-28 CELSIUS OUTFLOW: WEAK RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: LOW INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TD 25W IS FORECAST TO CONTINUE TRACKING WESTWARD FOR THE NEXT 24 HOURS ALONG THE SOUTHERN SIDE OF THE STEERING RIDGE ENTRENCHED TO THE NORTH. FORWARD SPEED IS EXPECTED TO BEGIN TO SLOW AFTER TAU 12 AS THE SYSTEM REACHES THE INFLECTION POINT BETWEEN THE STR CENTER TO THE NORTHEAST AND THE BUILDING CENTER TO THE WEST-NORTHWEST. BY TAU 36, THE RIDGE TO THE EAST WEAKENS AND SLIDES SOUTHWEST WHILE THE RIDGE TO THE WEST CONTINUES TO BUILD AND ASSUMES THE PRIMARY STEERING ROLE. TD 25W WILL TURN ONTO A SOUTHWESTWARD TRACK BY TAU 36, AND CONTINUE ON THIS TRAJECTORY THROUGH THE REMAINDER OF THE FORECAST. WHILE TD 25W CONTINUES TO UNDERPERFORM DUE TO THE MARGINAL CONDITIONS DESCRIBED ABOVE, GLOBAL MODEL FIELDS CONTINUE TO SUGGEST THAT THE ENVIRONMENT WILL IMPROVE, WITH THE STABLE AIR MASS TO THE WEST MODIFYING AND BECOMING LESS STABLE. VWS IS EXPECTED TO REMAIN LOW AND OUTFLOW WILL MARGINALLY IMPROVE AND THE HWRF HIGH RESOLUTION MODEL FIELDS ARE CONVINCED THE RESULT WILL BE A FAIRLY RAPID CONSOLIDATION OF THE INNER CORE OF THE SYSTEM BY TAU 24. IF THIS OCCURS AS THE MODELS SUGGEST, THE SYSTEM SHOULD SLOWLY INTENSIFY UP TO A PEAK OF 50 KNOTS BY TAU 36. BUT THE PARTY WON'T LAST LONG AND BY TAU 48, UPPER-LEVEL CONVERGENCE AND ANOTHER PUNCH OF DRY AIR IN THE MID-LEVELS WILL BEING TO SLOWLY SMOTHER THE SYSTEM, LEADING TO STEADY WEAKENING THROUGH THE REMAINDER OF THE FORECAST. THE SYSTEM EXPECTED TO MAKE LANDFALL IN THE VICINITY OF HUE, VIETNAM, BETWEEN TAUS 96 AND 120, BUT POTENTIALLY AS EARLY AS TAU 96, AS A WEAK TROPICAL DEPRESSION. ONCE ASHORE THE SYSTEM WILL RAPIDLY DISSIPATE NO LATER THAN TAU 120 OVER SOUTHERN LAOS. MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE REMAINS IN VERY GOOD AGREEMENT, WITH A MINIMAL SPREAD OF JUST 50NM AT TAU 48, THOUGH EVEN AT THIS EARLY STAGE, SOME ALONG TRACK SPREAD BEGINS TO REAR ITS HEAD. BY TAU 72, CROSS-TRACK SPREAD BETWEEN THE GFS TO THE NORTH AND UKMET ENSEMBLE TO THE SOUTH INCREASES TO 80NM BUT ALONG-TRACK SPREAD BETWEEN THE FAST ECMWF AND THE LAGGING HWRF INCREASES TO 160NM. BY TAU 120 THE SPREAD INCREASES DRASTICALLY AS THE MODELS BEGIN TO LOSE TRACK ON THE VORTEX, AND SPREAD INCREASES TO OVER 400NM, AND ALONG-TRACK SPREAD IS WELL ABOVE THAT, WITH THE HWRF AND GFS THINKING THEY ARE IN A FORMULA ONE RACE AND MOVING THE SYSTEM TO NEAR BANGKOK BY TAU 120 WHILE THE CONSENSUS MEAN REMAINS OVER LAOS. THE JTWC FORECAST LIES NORTH OF THE MEAN, BUT ROUGHLY FOLLOWS IT IN TERMS OF SPEED, THROUGH THE FORECAST WITH MEDIUM CONFIDENCE IN THE EARLY TAUS AND LOW CONFIDENCE IN THE EXTENDED TAUS. INTENSITY GUIDANCE IS IN GOOD AGREEMENT, WITH HWRF TAKING THE MOST AGGRESSIVE TACK, WITH A PEAK OF 60 KNOTS, THE BULK OF THE RELIABLE GUIDANCE PEAKING THE SYSTEM BETWEEN 40-50 KNOTS. THE JTWC FORECAST LIES CLOSE TO THE CONSENSUS MEAN THROUGH THE FORECAST WITH MEDIUM CONFIDENCE FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN