Advisories

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//
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