WDPN34 PGTW 130900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 27W (USAGI) WARNING NR 010// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 15.6N 126.5E INITIAL INTENSITY: 75 KTS GEOGRAPHIC REFERENCE: 324 NM EAST OF MANILA, PHILIPPINES MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 13 KTS SIGNIFICANT WAVE HEIGHT: 35 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A VERY COMPACT SYSTEM, WITH WELL DEFINED SPIRAL BANDING FEATURES AND A SMALL, SOMEWHAT ASYMMETRIC CENTRAL DENSE OVERCAST (CDO) FEATURE. THE FOCUS OF THE DEEP CONVECTION IS ON THE EASTERN SIDE OF THE CIRCULATION FOR THE TIME BEING, WITH THE NORTHWESTERN SIDE BEING DEVOID OF SIGNIFICANT CONVECTIVE ACTIVITY. A 13558 GMI 89GHZ MICROWAVE IMAGE AND A NEAR SIMULTANEOUS AMSR2 PASS REVEALED AN EXTREMELY SMALL INNER CORE OF CONVECTION, WITH WELL DEFINED, IF MORE SHALLOW, SPIRAL BANDS WRAPPING INTO THE CORE. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE MICROWAVE IMAGERY NOTED ABOVE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, HEDGED TOWARDS THE PGTW DVORAK CURRENT INTENSITY ESTIMATE OF T4.5 (77 KNOTS) AND THE CIMSS D-MINT. THE SMALL NATURE OF THE CORE MEANS THAT THE OBJECTIVE DVORAK-BASED ESTIMATES SUCH AS ADT, AIDT AND D-MINT ARE LIKELY A BIT TOO LOW. THE ENVIRONMENT REMAINS FAVORABLE WITH WARM SSTS, LOW VWS AND GOOD OUTFLOW ALOFT. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T4.5 - 77 KTS RJTD: T4.0 - 65 KTS KNES: T4.0 - 65 KTS CIMSS SATCON: 66 KTS AT 130600Z CIMSS ADT: 57 KTS AT 130530Z CIMSS AIDT: 64 KTS AT 130530Z CIMSS D-MINT: 79 KTS AT 130604Z CIMSS D-PRINT: 69 KTS AT 130530Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 10-15 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE RADIAL OTHER FACTORS: MODERATE EQUATORWARD OUTFLOW; SMALL SIZE OF THE CIRCULATION. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE FORECAST PHILOSOPHY HAS CHANGED SIGNIFICANTLY, WITH A CHANGE IN THE TAU 72 FORECAST POINT OF 230NM, THE TAU 96 FORECAST POINT OF 455NM AND THE TAU 120 FORECAST POINT OF 760NM. THE EXTREME UNCERTAINTY IN BOTH THE DETERMINISTIC (2100NM SPREAD) AND ENSEMBLE GUIDANCE (SIMILAR IF NOT GREATER) IS DRIVING THIS CHANGE, AS IT IS TOO EARLY TO MAKE A DEFINITIVE CALL ON WHICH SCENARIO WILL ULTIMATELY WIN OUT. FORECAST DISCUSSION: NO SIGNIFICANT CHANGE IS EXPECTED IN THE TRACK FORECAST FOR THE FIRST 24 HOURS, AND THE FORECAST REMAINS VERY CONSISTENT WITH THE PREVIOUS FORECAST THROUGH TAU 24. THE SYSTEM WILL TRACK NORTHWEST ALONG THE SOUTHWEST SIDE OF THE STRONG STR THROUGH THIS PERIOD. THE SYSTEM WILL BRUSH THE NORTHEASTERN TIP OF LUZON AROUND TAU 24 THEN MOVE INTO THE LUZON STRAIT BY TAU 36. AT THIS POINT, THE FORECAST BECOMES HIGHLY UNCERTAIN. WHAT MOST MODELS AGREE ON IS THAT THE STEERING RIDGE WILL MOVE THE EAST AND REORIENT SUCH THAT THE STEERING GRADIENT WEAKENS AND SHIFTS ONTO A MORE NORTH-SOUTH AXIS AS A DEEP TROF APPROACHES FROM THE WEST. THE MAIN DIFFERENCE BETWEEN THE GFS AND ECMWF (REPRESENTING THE TWO DISTINCT SIDES OF THE BIFURCATION) IS THE STRENGTH AND SIZE OF THE VORTEX AND THE IMPACT OF INCREASED SHEAR AND DRY AIR INTRUSION. THE GFS SHOWS THE SYSTEM 24MB WEAKER THAN THE ECMWF, WITH A VERY SMALL INNER CORE. THE GFS TAKES THE SYSTEM OVER LUZON AND EMERGES SIGNIFICANTLY WEAKENED. AS THE WEST-SOUTHWEST SHEAR PICKS UP AFTER TAU 48, IT HAS AN OUTSIZED IMPACT ON THE WEAKENED VORTEX IN THE GFS. THE RESULT IS A RAPID DECAPITATION AND WEAKENING OF THE SYSTEM, WHICH THEN TURNS SHARPLY SOUTHWESTWARD BY TAU 72, TRACKING FURTHER INTO THE SOUTH CHINA SEA ALONG THE NORTHERLY LOW-LEVEL FLOW. THE ECMWF ON THE OTHER HAND HAS A MUCH STRONGER AND LARGER VORTEX WHICH STAYS OVER WATER, SKIRTING THE TIP OF LUZON. THIS STRONGER AND LARGER VORTEX IS ABLE TO RESIST THE INCREASED SHEAR AFTER TAU 48, AT LEAST TO A BETTER DEGREE THAN THE GFS. THE SYSTEM TURNS NORTHWARD AND THEN RECURVES AROUND THE RIDGE AXIS, AHEAD OF THE APPROACHING TROF. AT THIS POINT, THE ULTIMATE TRACK THE SYSTEM TAKES WILL DEPEND ON THE HOW QUICKLY AND HOW MUCH IT INTENSIFIES AND THE EXACT TRACK OVER OR NEAR LUZON. THE JTWC FORECAST SLOWS THE SYSTEM DOWN AS IT MOVES INTO THE WEAKENED STEERING PATTERN, BUT GENERALLY FOLLOWS THE RECURVE SCENARIO. THE SYSTEM IS EXPECTED TO STEADILY INTENSIFY FOR THE NEXT 24 HOURS, THEN SLOWLY WEAKEN AS IT MOVES INTO RELATIVELY COOLER, LOWER OHC WATERS AND SLOWLY SUCCUMBS TO THE EFFECTS OF INCREASED SHEAR. MODEL DISCUSSION: AS MENTIONED IN THE FORECAST CHANGES PARAGRAPH ABOVE, THE GUIDANCE CAN ONLY BE DESCRIBED AS EXTREMELY UNCERTAIN, WITH A CAPITAL E. THE DETERMINISTIC MEMBERS OF THE CONSENSUS SHOW 2100NM OF SPREAD AT TAU 120, WITH THE GFS AND GEFS SHOWING THE REMNANT SYSTEM MAKING LANDFALL IN CENTRAL VIETNAM, WHILE THE EMCWF, EGRR AND GALWEM HAVE THE SYSTEM SOUTH OF SHIKOKU ISLAND. ENSEMBLE GUIDANCE SHOWS A SIMILAR DEGREE OF UNCERTAINTY BETWEEN THE MODELS. THE 0000Z ECENS RUN NOW SHOWS ALL ENSEMBLE MEMBERS SHOWING A RECURVE, OVER OKINAWA AND SOUTH OF JAPAN. THE GEFS 0000Z RUN IS 180DEG OUT, WITH ALL BUT TWO MEMBERS SHOWING A TURN SOUTHWEST AFTER TAU 48. THE REASONS FOR THE ENSEMBLE DIFFERENCES ARE THE SAME AS FOR THE DETERMINISTIC TRACKERS; THE ECENS IS STRONGER AND BIGGER AND MORE RESISTANT TO THE SHEAR AND DRY AIR, WHILE THE GEFS IS WEAKER AND SMALLER AND THUS MORE SUSCEPTIBLE TO THE IMPACTS OF THE SHEAR AND DRY AIR. WITH THE EXTREME UNCERTAINTY IN THE TRACK GUIDANCE, THE JTWC IS SET WITH BASICALLY ZERO CONFIDENCE AT THIS POINT. INTENSITY GUIDANCE IS SIMILARLY UNCERTAIN AFTER TAU 24, WITH THE ULTIMATE INTENSITY BEING HIGHLY DEPENDENT UPON WHICH TRACK THE SYSTEM TAKES AND THE JTWC FORECAST IS SET WITH VERY LOW CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: LOW TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: LOW INTENSITY 72-120 HR: LOW// NNNN