Advisories

WDPN32 PGTW 232100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 29W (CHAMPI) WARNING NR 038//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.6N 122.3E
   INITIAL INTENSITY: 95 KTS
   GEOGRAPHIC REFERENCE: 53 NM SOUTHEAST OF TAIPEI, TAIWAN
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 37 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TY 29W HAS STEADILY WEAKENED AND REMAINED NEARLY QUASI-STATIONARY
OVER THE PAST SIX HOURS DUE PRIMARILY TO LAND INTERACTION WITH
TAIWAN. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
DEEP CONVECTION COLLAPSING AS TY 29W CONTINUES TO INGEST SUBSIDENT
DOWNSLOPE WESTERLY FLOW OFF THE LEE SLOPES OF THE MOUNTAINOUS
TERRAIN OF TAIWAN. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON A GROUPING OF SUBJECTIVE GEOSTATIONARY
SATELLITE POSITION AND RADAR FIXES FROM VARIOUS REPORTING AGENCIES.
THE INITIAL INTENSITY OF 95 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON CONSENSUS OF SUBJECTIVE DVORAK AND AUTOMATED INTENSITY
ESTIMATES LISTED BELOW. GIVEN THE RAPID DEGRADATION IN STRUCTURE,
95 KNOTS REPRESENTS A CONSERVATIVELY HIGH-END ESTIMATE AT THIS
POINT.

INITIAL WIND RADII BASIS: 231245Z METOP-C ASCAT DATA AND OBJECTIVE
BEST TRACK.

CURRENT STEERING MECHANISM: ERRATIC MOTION ASSOCIATED WITH TERRAIN
INTERACTION OFFSETTING FLOW ALONG THE 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: T5.5 - 102 KTS
   CIMSS SATCON: 69 KTS AT 231743Z
   CIMSS ADT: 99 KTS AT 231730Z
   CIMSS AIDT: 101 KTS AT 231730Z
   CIMSS D-MINT: 91 KTS AT 231744Z
   CIMSS D-PRINT: 87 KTS AT 231830Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   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: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST TRACK LINE HAS SHIFTED
SLIGHTLY TO THE SOUTH FROM THE PREVIOUS WARNING. ADDITIONALLY,
EXTRATROPICAL TRANSITION OCCURS EARLIER, BEGINNING AT TAU 48 AND
COMPLETING BY TAU 60, IN THE CURRENT FORECAST.

FORECAST DISCUSSION: TY 29W IS EXPECTED TO REMAIN NEARLY
QUASI-STATIONARY OVER THE NEXT SEVERAL HOURS AS THE CIRCULATION
REORGANIZES FOLLOWING REMARKABLE STRUCTURAL CHANGE INDUCED BY
INTERACTION OF THE CIRCULATION WITH THE MOUNTAINOUS TERRAIN OF
TAIWAN. ADDITIONAL STEADY WEAKENING IS ANTICIPATED DURING THE
NEAR-TERM REORGANIZATION PHASE, DRIVEN BOTH BY INGESTION OF DRIER,
DOWN-SLOPING AIR FROM THE WEST AND LIKELY COOLING OF ALREADY
MARGINAL SEA SURFACE TEMPERATURES AS THE SYSTEM LOITERS TO THE EAST 
OF TAIWAN. BY TAU 12, THE DEEP-LAYER SUBTROPICAL RIDGE CENTERED TO 
THE SOUTH OF HONSHU WILL ONCE AGAIN PICK UP THE SYSTEM AND SEND IT ON 
A POLEWARD TRAJECTORY. BY TAU 48, THE SYSTEM WILL BEGIN EXTRATROPICAL 
TRANSITION AS IT INTERACTS WITH A DEVELOPING MID-LATITUDE TROUGH TO 
THE NORTHWEST. THE SYSTEM WILL QUICKLY MERGE WITH THIS TROUGH AND IS 
EXPECTED TO BECOME FULLY EMBEDDED BY TAU 60. STEADY WEAKENING IS 
EXPECTED PRIOR TO EXTRATROPICAL TRANSITION AS THE SYSTEM CONTENDS 
WITH INCREASING VERTICAL WIND SHEAR AND PASSAGE OVER COOLER WATER. 
AFTER TRANSITION, THE SYSTEM WILL FULLY MERGE WITH THE MID-LATITUDE
TROUGH AND INTENSITIES AFTER TAU 60 ARE REFLECTIVE OF THE MERGED
SYSTEM, WHICH IS EXPECTED TO STRENGTHEN OVER TIME UNDER FAVORABLE
BAROCLINIC SUPPORT.

MODEL DISCUSSION: NUMERICAL MODEL TRACK FORECAST GUIDANCE IS IN
GOOD AGREEMENT REGARDING THE OVERALL SCENARIO. THE DIFFERENCE IN
PREVIOUSLY NOTED ALONG-TRACK SPEEDS BETWEEN AI AND PHYSICS-BASED
MODELS HAS SIGNIFICANTLY DECREASED, BRINGING CONW AND AICN INTO
BETTER ALIGNMENT, WITH UNCERTAINTY PRIMARILY CONFINED TO TRACK
DIRECTION. THE CURRENT JTWC FORECAST HAS SHIFTED TO THE SOUTH FROM
THE PREVIOUS FORECAST IN LINE WITH A SHIFT IN BOTH CONW AND AICN.
NOTABLY, AI MODELS DEPICT A TRACK FARTHER TO THE SOUTH OF HONSHU.
ADDITIONAL ADJUSTMENTS IN TRACK ORIENTATION ARE POSSIBLE DURING
SUBSEQUENT CYCLES GIVEN BOTH MODEL SPREAD AND UNCERTAINTY
ASSOCIATED WITH NEAR-TERM MOTION. INTENSITY FORECAST GUIDANCE IS IN
GOOD AGREEMENT THROUGHOUT THE FORECAST PERIOD, WITH THE INITIAL
INTENSITY FACTORING IN AS THE LARGEST ELEMENT OF UNCERTAINTY EARLY
ON AND INTERACTION WITH THE MID-LATITUDE TROUGH DURING AND AFTER
EXTRATROPICAL TRANSITION CREATING UNCERTAINTY IN THE EXTENDED
PERIOD.

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