Advisories

WDPN33 PGTW 291500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (BANG-LANG) WARNING NR 014//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.3N 168.6E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 377 NM NORTH-NORTHEAST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 26 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
THE CLOUD PATTERN OF TYPHOON (TY) 20W HAS BECOME LESS ORGANIZED.
THE NASCENT EYE FEATURE THAT WAS PREVIOUSLY VISIBLE IS NO LONGER
APPARENT. WHILE VERY DEEP CONVECTION CONTINUES TO FLARE ALONG THE
NORTHERN AND EASTERN PERIPHERIES, WITH OVERSHOOTING TOPS AS COLD AS
-85 DEGREES CELSIUS, THE OVERALL STRUCTURE HAS DEGRADED. THE
MODERATE TO HIGH VERTICAL WIND SHEAR (VWS) OF 20-25 KTS HAS TAKEN A
TOLL ON THE CONVECTIVE SIGNATURE, RESTRICTING UPPER-LEVEL OUTFLOW
ON THE WESTERN SIDE OF THE SYSTEM. FURTHERMORE, TY 20W IS NOW
TRAVERSING THE COOL WAKE LEFT BEHIND BY TROPICAL STORM (TS) 21W,
WITH SEA SURFACE TEMPERATURES (SST) OF 27-28 DEGREES CELSIUS, A FEW
DEGREES COOLER THAN 12 HOURS AGO. THE OCEAN, HOWEVER, IS STILL
SUFFICIENTLY WARM TO MAINTAIN A TROPICAL CYCLONE. THEREFORE, THE
ENVIRONMENT IS MARGINALLY UNFAVORABLE. A 290641Z RCM-1 SAR PASS
ESTIMATED MAXIMUM SUSTAINED WINDS OF 80-85 KTS, WHICH WAS ASSESSED
TO BE A SLIGHT OVERESTIMATE. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON THE MID-LEVEL CIRCULATION CENTER
OBSERVED IN THE EIR IMAGERY. THE INITIAL INTENSITY IS HELD AT 75
KTS BASED ON THE EARLIER SAR DATA AND THE T4.5 FIXES FROM PGTW,
PHFO, AND KNES. THIS ASSESSMENT IS NOTED TO BE WELL ABOVE THE
CURRENT OBJECTIVE ESTIMATES FROM CIMSS.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: WESTERN FLANK OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED NEAR THE DATELINE

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T4.0 - 65 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T4.5 - 77 KTS
   PHFO: T4.5 - 77 KTS
   CIMSS SATCON: 59 KTS AT 290530Z
   CIMSS ADT: 51 KTS AT 291200Z
   CIMSS AIDT: 46 KTS AT 291200Z
   CIMSS D-MINT: 66 KTS AT 291113Z
   CIMSS D-PRINT: 61 KTS AT 291200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE EASTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE INTENSITY FORECAST HAS BEEN
LOWERED 10-15 KTS ACROSS THE BOARD.

FORECAST DISCUSSION: TY 20W WILL MAINTAIN A POLEWARD TRAJECTORY
THROUGHOUT THE FORECAST PERIOD, STEERED BY AN EXTENSIVE RIDGE OVER
THE CENTRAL NORTH PACIFIC. THE SYSTEM WILL TAKE A SLIGHT BEND
NORTHWESTWARD FROM TAU 12 TO TAU 36, AS IT BRIEFLY INTERACTS WITH
THE CIRCULATION OF TS 21W. BY TAU 48, AN APPROACHING LOW- TO
MID-LEVEL TROUGH WILL INDUCE A NORTHEASTWARD MOTION. BASED ON THIS
FORECAST TRACK, THE STORM WILL RAPIDLY WEAKEN. THE VWS WILL ENTRAIN
INCREASING AMOUNTS OF DRY AIR INTO THE MINIATURE VORTEX. THE
CIRCULATION WILL MOVE OVER PROGRESSIVELY COOLER SST, CROSSING THE
26 DEGREE ISOTHERM BY TAU 24. THESE FACTORS WILL RESULT IN A RAPID
WEAKENING TREND AND A TRANSITION TO A SUBTROPICAL STORM BY TAU 48.

MODEL DISCUSSION: MODELS REMAIN IN FAIR AGREEMENT THROUGH THE
FORECAST, WITH BOTH A CROSS-TRACK AND ALONG-TRACK SPREAD OF
APPROXIMATELY 140 NM AT TAU 48. THE GUIDANCE ENVELOPE THROUGH TAU
24 IS COMPRISED OF A TIGHT CORE OF PHYSICS-BASED AND AI MODELS WITH
A FEW OUTLIERS ALONG THE WESTERN EDGE. THE JTWC TRACK FORECAST IS
POSITIONED ALONG THIS CORE GROUP OF MODELS, WHICH LIES ALONG THE
EASTERN SIDE OF THE ENVELOPE FOR THE FIRST 24 HOURS AND THEN LEANS
TOWARD A BLEND OF THE AI CONSENSUS AND THE JTWC CONSENSUS (CONW).
THE RELIABLE INTENSITY AIDS, INCLUDING HAFS-A AND GFS, DEPICT RAPID
WEAKENING THROUGH TAU 48. THE JTWC INTENSITY FORECAST IS ALIGNED
ACCORDINGLY, SET ALONG THE LOWER END OF THE SPREAD.

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