WDPN31 PGTW 241500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 25W (SURIGAE) WARNING NR 006// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 18.6N 132.5E INITIAL INTENSITY: 40 KTS GEOGRAPHIC REFERENCE: 541 NM SOUTH-SOUTHEAST OF KADENA AB MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS SIGNIFICANT WAVE HEIGHT: 19 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED SHORTWAVE INFRARED (SWIR) AND PROXY VISIBLE (PROXYVIS) SATELLITE IMAGERY DEPICTS A FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) DISPLACED TO THE SOUTHEAST OF AN AREA OF ROBUST CONVECTION AND MID-LEVEL ROTATION. A 240912Z WSF-M COLOR-ENHANCED 89GHZ MICROWAVE IMAGE WAS THE FIRST PIECE OF DATA TO REVEAL THE ONSET OF RAPID VORTEX DECOUPLING, WITH THE LLCC HAVING EMERGED FROM THE SOUTHEASTERN EDGE OF THE CONVECTIVE MASS. A 241212Z ASCAT-B PASS PROVIDED ADDITIONAL EVIDENCE THAT THE VORTEX HAD BY THE 1200Z HOUR, COMPLETELY DECOUPLED AND BECOME EXPOSED, THOUGH STILL UNDER THIN CIRRUS, UNDER THE INFLUENCE OF MODERATE TO HIGH SOUTHERLY VERTICAL WIND SHEAR. THE ASCAT DATA ALSO DEPICTED A MUCH WEAKER WIND FIELD THAN EXPECTED, WITH MAXIMUM WINDS OF 40 KNOTS TO THE NORTH UNDER THE CONVECTION, AND THE BULK OF THE WINDS BEING 30-35 KNOTS. THE WIND FIELD REMAINS HIGHLY ASYMMETRIC AS WELL, WITH MAXIMUM WINDS ACROSS THE SOUTHERN PORTION OF THE CIRCULATION RESTRICTED TO JUST 15-20 KNOTS. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE ASCAT-B PASS DISCUSSED ABOVE. THE INITIAL INTENSITY IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE SAME SCATTEROMETER DATA, SET SLIGHTLY LOWER THAN THE MAJORITY OF THE SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES OUTLINED BELOW. ENVIRONMENTAL CONDITIONS ARE MARGINALLY FAVORABLE, CHARACTERIZED BY VERY WARM SSTS, HIGH OHC, AND MODERATE POLEWARD OUTFLOW OFFSET BY MODERATE TO STRONG SOUTHERLY SHEAR, DRY AIR ENTRAINMENT AND SIGNIFICANT VORTEX DECOUPLING. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHRY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED SOUTH OF HONSHU. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS RJTD: T2.5 - 35 KTS RCTP: T2.5 - 35 KTS CIMSS ADT: 49 KTS AT 241140Z CIMSS AIDT: 44 KTS AT 241140Z CIMSS D-PRINT: 44 KTS AT 241210Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 15-20 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE POLEWARD OTHER FACTORS: VORTEX DECOUPLING; DRY AIR ENTRAINMENT. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: 25W HAS BEEN RELOCATED APPROXIMATELY 55NM SOUTHEAST FROM ITS EXPECTED 1200Z POSITION BASED ON RECEIPT OF HIGH-RESOLUTION MICROWAVE AND SCATTEROMETER DATA. THE FORECAST TRACK HAS ALSO SHIFTED EASTWARD AFTER TAU 60, INCREASING THE CPA TO OKINAWA TO 90NM. FORECAST DISCUSSION: TROPICAL STORM (TS) 25W HAS BEEN RELOCATED BASED ON RECENT MICROWAVE AND SCATTEROMETER DATA WHICH DEFINITIVELY ESTABLISHED THAT THE SYSTEM VORTEX HAS FULLY DECOUPLED. THIS HAS RESULTED IN A SLIGHTLY MORE WEST-NORTHWESTWARD TRACK, BUT MORE IMPORTANTLY HAS SLOWED THE TRACK SPEED BY NEARLY HALF, FROM 16 KNOTS TO 9 KNOTS. THE MID-LEVEL VORTEX IS EMBEDDED WITHIN THE CENTRAL DENSE OVERCAST (CDO) FEATURE TO THE NORTHWEST OF THE LLCC. WHILE THE SYSTEM HAS DECOUPLED, THE OVERALL SYNOPTIC STEERING PATTERN HAS NOT CHANGED AND THE FORECAST CALLS FOR 25W TO CONTINUE TRACKING NORTHWESTWARD FOR THE NEXT 36 HOURS AT A STEADY 9-10 KNOTS. BY TAU 36, THE SYSTEM WILL BEGIN TO DECELERATE AS IT APPROACHES A BREAK IN THE RIDGE AND REACHES THE RIDGE AXIS BY TAU 48. HOWEVER, RIDGE AXIS IN THIS CASE IS LOOSELY DEFINED AS THE STR ITSELF IS EXTENDED ALONG THE NORTH-SOUTH AXIS, WITH A VERY LARGE COL REGION BETWEEN IT AND A SECONDARY RIDGE CENTER OVER SOUTHERN CHINA. SO WHILE 25W WILL MOVE INTO THE COL REGION AND BEGIN TO RECURVE AFTER TAU 48, TRACK SPEEDS WILL REMAIN SLOW THROUGH TAU 96 AS IT WILL REMAIN UNDER THE INFLUENCE OF A RELATIVELY WEAK STEERING GRADIENT. THE JTWC FORECAST CALLS FOR THE SYSTEM TO BE PULLED NORTHWARD BY TWO FACTORS; FIRSTLY THE STR SHIFTS NORTH AND EAST AND SECOND, THE PASSAGE OF A MID-LATITUDE TROUGH TO THE NORTH. BY TAU 120, 25W WILL BEGIN TO ACCELERATE NORTHEASTWARD AS IT STARTS ITS RECURVE. THERE REMAINS CONSIDERABLE UNCERTAINTY IN THE FORECAST, ESPECIALLY AFTER TAU 48, WHEN THE SYSTEM MOVES INTO THE WEAK STEERING PATTERN ASSOCIATED WITH THE COL REGION. THE KEY DETERMINANT IN WHETHER 25W RECURVES OR NOT WILL BE THE STRENGTH OF THE TROUGH TO THE NORTH. THE ECMWF AND AI MODELS SHOW A SLIGHTLY DEEPER TROUGH, WITH ENOUGH SUPPORT TO PULL 25W POLEWARD AND THE JTWC FORECAST IS BASED ON THIS SCENARIO. THE ALTERNATE SCENARIO, AS DEPICTED BY THE GFS AND TO VARYING DEGREES BY ALL OF THE OTHER DETERMINISTIC MODELS EXCEPT ECMWF, SHOWS A WEAKER TROUGH WITH LIMITED DYNAMIC SUPPORT, INSUFFICIENT TO BRING 25W POLEWARD, RESULTING IN 25W BECOMING QUASI-STATIONARY IN THE VICINITY OF THE SOUTHERN RYUKYUS BY TAU 120. REGARDING INTENSITY, THE SYSTEM HAS WEAKENED DUE TO THE VORTEX DECOUPLING BROUGHT ON BY PERSISTENT SOUTHERLY SHEAR. GLOBAL MODELS ARE IN AGREEMENT THAT SHEAR WILL ABATE OVER THE NEXT 24 HOURS, AND THE SYSTEM WILL GRADUALLY ACHIEVE VORTEX ALIGNMENT ONCE THE SHEAR DECREASES. ASSUMING 25W ACHIEVES AXISYMMETRIZATION BY TAU 36, IT SHOULD RAPIDLY INTENSIFY (RI) BETWEEN TAU 36 AND TAU 60 TO A PEAK OF 100 KNOTS. UPON COMMENCING ITS RECURVE, 25W WILL GRADUALLY WEAKEN THROUGH THE END OF THE FORECAST PERIOD UNDER DETERIORATING ENVIRONMENTAL CONDITIONS OF INCREASED SHEAR AND DRY AIR ENTRAINMENT. HOWEVER, THE INTENSITY FORECAST IS HIGHLY DEPENDENT UPON THE TRACK FORECAST AND IN THE ALTERNATE SCENARIO, 25W WILL LIKELY ACHIEVE A HIGHER PEAK INTENSITY ROUGHLY 24 HOURS LATER. MODEL DISCUSSION: TRACK GUIDANCE IS IN DECENT AGREEMENT IN THE SHORT-TERM, EVEN IN LIGHT OF THE RECENT RELOCATION. EVEN AS EARLY AS TAU 48 HOWEVER, THE MODELS BEGIN TO DIVERGE, WITH THREE CLEAR GROUPINGS BECOMING EVIDENT. THE FIRST GROUP CONSISTS OF THE NAVGEM, GALWEM AND EGRR, WHICH INSIST ON TAKING THE SYSTEM STRAIGHT NORTHWESTWARD, ULTIMATELY REACHING A POSITION NORTHWEST OF MIYAKOJIMA BY TAU 120. THE OTHER GROUP CONSISTS OF THE AI MODELS, WHICH BEGIN TO OUTPACE THE PHYSICS-BASED GUIDANCE AS EARLY AS TAU 48, INCREASING THEIR ACCELERATION THROUGH TAU 120 SUCH THAT BY THE END OF THE FORECAST, THEY ARE LOCATED IN A RANGE ROUGHLY FROM A POSITION EAST OF AMAMI OSHIMA ALL THE WAY TO SOUTH OF YOKOSUKA. THIS RESULTS IN AN ALONG-TRACK SPREAD OF OVER 750NM. THE REMAINDER OF THE GUIDANCE MAKES UP THE THIRD GROUP CONSISTING OF THE GFS, GEFS, ECMWF, ECEPS AND THE CONSENSUS MEAN, WHICH DO SHOW A RECURVE SCENARIO, BUT AT A MUCH SLOWER PACE, ROUGHLY POSITIONED EAST OF OKINAWA BY TAU 120. ADDITIONALLY, THE ENTIRE GUIDANCE PACKAGE HAS SHIFTED EASTWARD NEARLY 100NM ON THE 12Z RUN. THE JTWC EXPERIMENTAL SUPER-ENSEMBLE HAS ALSO SHIFTED EASTWARD WHILE BECOMING MORE ALIGNED WITH THE AI MODELS IN TERMS OF TRACK SPEED. THE JTWC FORECAST IS PLACED NEAR THE AI CONSENSUS MEAN THROUGH TAU 60, THEN A BLEND BETWEEN THE CONSENSUS AND AI CONSENSUS THEREAFTER. CONFIDENCE IS LOW THROUGHOUT THE FORECAST PERIOD. INTENSITY GUIDANCE ALSO DEPICTS A LARGE AMOUNT OF UNCERTAINTY, WITH PEAK INTENSITIES RANGING BETWEEN 65 KNOTS AND 155 KNOTS. THE BULK OF THE GUIDANCE SHOWS A PEAK BETWEEN TAU 60 AND TAU 72, CONSISTENT WITH THE TRACK SPEED UNCERTAINTY IN THE TRACK GUIDANCE. THE CTCX AND HAFS-A DEPICT A HIGHER PEAK NEAR 110-120KTS ABOUT 12 HOURS LATER (TAU 72). THE BULK OF THE GUIDANCE DEPICTS A PERIOD OF RI BETWEEN TAU 36 AND TAU 60 UP TO THE PEAK INTENSITY, FOLLOWED BY A GRADUAL WEAKENING THROUGH THE END OF THE FORECAST. RIDE AND CTR1 ARE THE ONLY RI AIDS WHICH TRIGGERED ON THIS RUN, HOWEVER ENSEMBLE BASED RI PROBABILITIES REMAIN ELEVATED AND ASSUMING VORTEX ALIGNMENT, THE ENVIRONMENT SHOULD SUPPORT RI AFTER TAU 36. THE JTWC FORECAST IS PLACED NEAR THE OPERATIONAL GOOGLE DEEPMIND, THOUGH CONFIDENCE REMAINS LOW DUE TO THE TIGHT COUPLING OF THE INTENSITY AND TRACK FORECASTS AND THE HIGH UNCERTAINTY IN THE TRACK FORECAST ITSELF. FORECAST CONFIDENCE: TRACK 00-72 HR: LOW TRACK 72-120 HR: LOW INTENSITY 00-72 HR: LOW INTENSITY 72-120 HR: LOW// NNNN