WDPN33 PGTW 300900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (BANG-LANG) WARNING 
NR 017//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 33.3N 167.3E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 841 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 26 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A RAPIDLY
DETERIORATING CONVECTIVE STRUCTURE ASSOCIATED WITH TROPICAL STORM
(TS) 20W. IN THE PAST 24 HOURS, THE CENTRAL DENSE OVERCAST AND
NASCENT EYE FEATURE QUICKLY DEVOLVED INTO A CONVECTIVE BAND
ISOLATED TO THE NORTHERN PERIPHERY. THE CIRCULATION IS BECOMING
EMBEDDED IN A LARGE CLOUD BAND, UNDER WHICH THERE IS AN EXTENSIVE
SOUTH-SOUTHEASTERLY WIND FETCH OF 20-30 KTS. THE HIGHLY COMPACT
LOW-LEVEL CIRCULATION CENTER (LLCC) IS FULLY OBSCURED BY DEEP
CONVECTION AND THE SURROUNDING MID- TO UPPER-LEVEL CLOUDS. THE
ENVIRONMENT IS UNFAVORABLE, CHARACTERIZED BY STRONG POLEWARD
OUTFLOW THAT IS BEING OFFSET BY COOL SEA SURFACE TEMPERATURES
(SST). THE SUSTAINED CONVECTION IS THE RESULT OF COLDER AIR IN THE
UPPER TROPOSPHERE AND STRONG DIVERGENCE ALOFT. A 300425Z SSMIS PASS
REVEALED A HOOK-SHAPED RAIN BAND ALONG THE NORTHERN PERIPHERY. THE
INITIAL POSITION IS ASSESSED WITH LOW CONFIDENCE BASED ON THE
EXTRAPOLATION OF THE PREVIOUSLY VISIBLE, PARTIALLY EXPOSED LLCC AND
A LARGE SPREAD AMONG THE AGENCY FIXES. THE INITIAL INTENSITY HAS
BEEN LOWERED TO 45 KTS WITH LOW CONFIDENCE, SUPPORTED BY A BLEND OF
THE OBJECTIVE ESTIMATES FROM CIMSS. THE LOWER FINAL T-NUMBERS FROM
THE SUBJECTIVE DVORAK ESTIMATES OF 2.5-3.5 SUPPORT THE LOWER
INTENSITY.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: BINARY INTERACTION WITH TROPICAL
DEPRESSION (TD) 21W AND THE SOUTHWESTERN FLANK OF AN EXTENSIVE
RIDGE POSITIONED OVER THE CENTRAL NORTH PACIFIC

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T3.5 - 55 KTS
   RCTP: T3.5 - 55 KTS
   KNES: T4.0 - 65 KTS
   CIMSS SATCON: 43 KTS AT 300340Z
   CIMSS ADT: 39 KTS AT 300610Z
   CIMSS AIDT: 43 KTS AT 300610Z
   CIMSS D-MINT: 37 KTS AT 300516Z
   CIMSS D-PRINT: 34 KTS AT 300610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 10-15 KTS
   SST: 25-26 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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: A MID-LEVEL RIDGE POSITIONED EAST OF THE
DATELINE IS THE PRIMARY STEERING MECHANISM OF TS 20W, BRINGING THE
SYSTEM POLEWARD AT A FORWARD SPEED OF 25-30 KTS. OVER THE PAST 6-12
HOURS, THE SYSTEM HAS TURNED SLIGHTLY WEST OF NORTH DUE TO THE
INTERACTION WITH TD 21W LOCATED APPROXIMATELY 180 NM TO THE
SOUTHWEST. TS 20W WILL PULL AWAY FROM TD 21W IN THE NEXT 12 HOURS,
AND THE STORM MOTION WILL RESUME A POLEWARD TRAJECTORY. A TROUGH
PASSING TO THE NORTH WILL IMPART A NORTH-NORTHEAST MOTION BY TAU
36. THE CIRCULATION CROSSED THE 26 DEGREE CELSIUS ISOTHERM 06 HOURS
AGO AND WILL TRAVERSE OVER GRADUALLY COOLER WATERS THROUGH THE
FORECAST PERIOD. THIS WILL LEAD TO GRADUAL WEAKENING THROUGH TAU 24
AS THE SYSTEM UNDERGOES SUBTROPICAL TRANSITION. IT WILL BECOME A
SUBTROPICAL STORM BY TAU 36. THE PROXIMITY OF TS 20W TO TD 21W HAS
INTRODUCED NEW FORECAST UNCERTAINTY. THERE IS NOW A DISTINCT
POSSIBILITY THAT TS 20W COULD MERGE WITH TD 21W. UNDER THIS
SCENARIO, THE CIRCULATION WOULD CONTINUE TO BEND TOWARD THE WEST
AND EVENTUALLY SOUTH. STEADY WEAKENING WOULD OCCUR UNTIL THE TWO
CIRCULATIONS MERGE, AFTER WHICH TIME THE END RESULT WOULD LIKELY BE
A TROPICAL DISTURBANCE OR DEPRESSION.

MODEL DISCUSSION: THE MAJORITY OF THE MODEL TRACKERS ARE ALIGNED
WITH A POLEWARD MOTION OVER THE NEXT 36 HOURS. HAFS-A IS THE
OUTLIER, DEPICTING A MERGER SCENARIO WITH TD 21W. BOTH THE
CROSS-TRACK AND ALONG-TRACK SPREADS AMONG THE GUIDANCE ARE QUITE
LARGE, CAUSED BY DIFFERING LEVELS OF INTERACTION WITH TD 21W AND A
TROUGH TO THE NORTHWEST. THE JTWC FORECAST TRACK IS PLACED WITH
MEDIUM CONFIDENCE NEAR THE AI CONSENSUS, WHICH LIES WITHIN THE
TIGHTEST GROUPING OF MODELS, THROUGH TAU 36. THE EXPERIMENTAL
GOOGLE DEEPMIND MODEL LOSES TRACK OF THE CIRCULATION BY TAU 12,
WHILE GFS DEPICTS A STEADY TREND AND HAFS-A STEADILY WEAKENS THE
STORM. THE JTWC INTENSITY FORECAST IS SET WITH MEDIUM CONFIDENCE
NEAR THE CENTER OF THE SPREAD AND IS CLOSEST TO THE MULTI-MODEL
CONSENSUS.

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