Advisories

WDPN32 PGTW 231500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 29W (CHAMPI) WARNING NR 037//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.0N 122.7E
   INITIAL INTENSITY: 100 KTS
   GEOGRAPHIC REFERENCE: 290 NM WEST-SOUTHWEST OF KADENA AB
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 38 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TYPHOON
(TY) 29W UNDERGOING RAPID STRUCTURAL DEGRADATION, QUICKLY LOSING ITS 
EYE AND EXHIBITING PRONOUNCED TROCHOIDAL MOTION AS IT INTERACTS
WITH THE TOPOGRAPHY OF NORTHERN TAIWAN. THE WELL-DEFINED EYE PASSED
JUST EAST OF YONAGUNI ISLAND AROUND 230930Z, WHERE THE SURFACE 
STATION REPORTED MAXIMUM WIND GUSTS OF 89 KTS AND A MINIMUM MEAN SEA
LEVEL PRESSURE (MSLP) OF 948.9 MB. IMMEDIATELY FOLLOWING PASSAGE OF
YONAGUNI ISLAND, THE EYE BEGAN TO FILL, DISAPPEARING IN EIR IMAGERY
WITHIN ONE HOUR. TY 29W ALSO INITIATED A LARGE NORTHWESTWARD 
TROCHOIDAL WOBBLE, CONSISTENT WITH EARLIER HAFS-A AND HWRF GUIDANCE,
AND THE CIRCULATION CENTER IS NOW POSITIONED JUST OFF THE EXTREME
NORTHEASTERN TIP OF TAIWAN. TOPOGRAPHIC INTERACTION WITH THE CENTRAL 
MOUNTAIN RANGE (CMR) IS THE PRIMARY DRIVER OF BOTH STRUCTURAL AND 
TRACK CHANGES. WARM, DRY, SUBSIDENT DOWNSLOPE WESTERLY FLOW OFF THE 
LEE SLOPES OF THE CMR ADVECTED DIRECTLY INTO THE CIRCULATION OF TY 
29W, RAPIDLY ERODING THE WESTERN SEMICIRCLE OF THE INNER CORE. THE 
TROCHOIDAL DEFLECTION IS LIKEWISE A DIRECT DYNAMICAL RESPONSE TO 
TERRAIN INTERACTION, A WELL-DOCUMENTED PHENOMENON IN THIS REGION. THE 
EYE, WHILE RAPIDLY EXPANDING AND BECOMING INCREASINGLY RAGGED, 
REMAINS WELL DEFINED IN ANIMATED DOPPLER RADAR DATA FROM THE CENTRAL 
WEATHER ADMINISTRATION (CWA) IN TAIWAN, SUPPORTING HIGH CONFIDENCE IN 
THE INITIAL POSITION. THE INITIAL INTENSITY IS ASSESSED WITH LOW 
CONFIDENCE, AS SOMEWHAT PARADOXICALLY, OBJECTIVE INTENSITY ESTIMATES 
HAVE RISEN DURING EYE DECAY WHILE SUBJECTIVE DVORAK DATA-T VALUES 
HAVE FALLEN SHARPLY. ENVIRONMENTAL CONDITIONS REMAIN SYNOPTICALLY 
FAVORABLE; HOWEVER, LAND PROXIMITY, ONGOING LOW-ENTHALPY DRY AIR 
ENTRAINMENT INTO THE INNER CORE, AND TRANSLATION OVER COOLER UPWELLED 
WATERS RENDER THE MESOSCALE ENVIRONMENT MARGINAL.

INITIAL WIND RADII BASIS: PERSISTENCE AND OBJECTIVE BEST TRACK. 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED SOUTH OF HONSHU

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   RJTD: T5.5 - 102 KTS
   RCTP: T5.0 - 90 KTS
   CIMSS ADT: 117 KTS AT 231130Z
   CIMSS AIDT: 102 KTS AT 231200Z
   CIMSS D-MINT: 105 KTS AT 231026Z
   CIMSS D-PRINT: 104 KTS AT 231200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 10-15 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: INGESTION OF DRY, SUBSIDENT AIR INTO THE CORE DUE
TO TERRAIN INTERACTION. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: LOW
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TY 29W IS EXHIBITING HIGHLY ERRATIC MOTION AS
IT WOBBLES TOWARD THE NORTHEASTERN TIP OF TAIWAN. TYPICAL TROCHOIDAL
OSCILLATIONS PERSIST FOR ROUGHLY 6-12 HOURS, AND TY 29W MAY EXECUTE A
SMALL CYCLONIC LOOP, A COMMON OCCURRENCE FOR TYPHOONS IN THIS
REGIME. REGARDLESS, TY 29W SHOULD RESUME A POLEWARD TRACK BY TAU 12. 
THE OFFICIAL FORECAST TRACK SMOOTHS THESE SHORT-TERM MESOSCALE 
PERTURBATIONS INTO AN INTEGRATED 12-HOUR TRAJECTORY. ALL AVAILABLE 
NUMERICAL GUIDANCE, INCLUDING MESOSCALE MODELS, SUPPORTS A RESUMPTION 
OF POLEWARD MOTION BY TAU 12, FOLLOWED BY NORTHEASTWARD ACCELERATION 
AFTER TAU 24 AS AN APPROACHING MID-LATITUDE SHORTWAVE TROUGH ERODES 
THE NORTHWESTERN PERIPHERY OF THE STR. BY TAU 48, TY 29W WILL BECOME 
EMBEDDED IN STRONG SOUTHWESTERLY MID-LATITUDE FLOW AND ACCELERATE
EAST-NORTHEASTWARD THROUGH THE REMAINDER OF THE FORECAST PERIOD. 
LANDFALL ALONG THE WESTERN COAST OF KYUSHU IS FORECAST BETWEEN TAU 60 
AND TAU 72, AFTER WHICH THE SYSTEM WILL RE-EMERGE OVER THE OPEN 
WATERS SOUTH OF SHIKOKU AND HONSHU THROUGH TAU AS THE SYSTEM 
APPROACHES KYUSHU AND PHASES WITH THE 500 MB SHORTWAVE TROUGH, IT 
WILL INITIATE EXTRATROPICAL TRANSITION (ETT), COMPLETING 
TRANSFORMATION BETWEEN TAU 72 AND TAU 96. REGARDING INTENSITY, RAPID 
INNER-CORE EROSION DRIVEN BY TERRAIN-INDUCED LOW-ENTHALPY AIR 
ENTRAINMENT WILL STEADILY WEAKEN THE SYSTEM OVER THE NEXT 12 HOURS. 
AS TY 29W TRACKS FURTHER POLEWARD, INCREASING WESTERLY VERTICAL WIND 
SHEAR (VWS) AND AN INFLUX OF DRY MID-LEVEL AIR FROM THE WEST WILL 
MAINTAIN A STEADY WEAKENING TREND THROUGH TAU 48. AS THE SYSTEM 
UNDERGOES ETT, THE WEAKENING RATE WILL MODERATE, AND THE VORTEX IS 
FORECAST TO SLIGHTLY RE-INTENSIFY UPON COMPLETING ETT DUE TO ROBUST 
BAROCLINIC FORCING OVER THE WARM KUROSHIO CURRENT. 

MODEL DISCUSSION: NUMERICAL TRACK GUIDANCE IS IN STRONG AGREEMENT ON 
THE SYNOPTIC RECURVATURE SCENARIO. EC-AIFS IS THE SOLE REMAINING 
MODEL DEPICTING NEAR-TERM ERRATIC MOTION, WHILE BOTH HAFS-A AND HWRF, 
WHICH CORRECTLY PREDICTED THE TROCHOIDAL MOTION SIX HOURS AGO, HAVE 
NOW SMOOTHED OUT THE MESOSCALE WOBBLE, YIELDING A DIRECT NORTHWARD 
TRAJECTORY SIMILAR TO THE REMAINDER OF THE GUIDANCE CONSENSUS. THE 
ECMWF, GALWEM, AND GFS ANCHOR THE WESTERN EDGE OF THE TRACK ENVELOPE, 
WHILE THE AI MODELS AND AI CONSENSUS (AICN) BOUND THE EASTERN EDGE, 
CONSISTENT WITH RECENT CYCLES. CROSS-TRACK SPREAD INCREASES TO 115 NM 
BY TAU 60 AND 330 NM BY TAU 96. CONSISTENT WITH PREVIOUS MODEL 
CYCLES, HOWEVER, ALONG-TRACK TIMING SPREAD IS A GREATER CONCERN, 
REACHING 250 NM BY TAU 60 BETWEEN THE SLOWER ECMWF AND FASTER AICN. 
BY TAU 96, ALONG-TRACK SPREAD BECOMES EXTREME, EXCEEDING 1000 NM 
BETWEEN THE PROGRESSIVE OUTLIERS (GFS EAST OF HOKKAIDO) AND GOOGLE 
DEEPMIND (GDM), WHICH DISSIPATES THE VORTEX SOUTH OF SASEBO. THE JTWC 
TRACK FORECAST IS PLACED ON THE WESTERN EDGE OF THE TRACK ENVELOPE 
THROUGH TAU 24 TO ACCOUNT FOR THE NEAR-TERM TOPOGRAPHIC DEFLECTION, 
THEN TRACKS NEAR THE AICN THROUGH THE REMAINDER OF THE FORECAST. 
INTENSITY GUIDANCE IS IN GOOD AGREEMENT, WITH ALL MODELS INDICATING 
STEADY TO RAPID WEAKENING THROUGH TAU 72, FOLLOWED BY STABILIZATION 
AND BAROCLINIC RE-INTENSIFICATION THROUGH THE END OF THE FORECAST 
PERIOD DURING ETT.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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