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JTWC/21W/#04/09-04 06Z Prognostic Reasoning
WDPN31 PGTW 040900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (PEIPAH) WARNING NR
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
INITIAL POSITION: 31.0N 131.5E
INITIAL INTENSITY: 35 KTS
GEOGRAPHIC REFERENCE: 486 NM WEST-SOUTHWEST OF YOKOSUKA, JAPAN
MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 15 KTS
SIGNIFICANT WAVE HEIGHT: 18 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A FULLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH BUILDING
CONVECTION ALONG THE NORTHERN PERIPHERY. THE CIRCULATION HAS
TRACKED NORTHWARD, JUST OFF THE COAST OF SOUTHEASTERN KYUSHU. AN
89GHZ GMI MICROWAVE IMAGE REVEALS WEAK LOW-LEVEL WRAPPING WITH DEEP
CONVECTION CONFINED TO THE NORTHERN SEMI-CIRCLE. THE ENVIRONMENT IS
ASSESSED TO BE MARGINALLY FAVORABLE, CHARACTERIZED BY LOW TO
MODERATE (15-20 KTS) VERTICAL WIND SHEAR (VWS), VERY WARM (30-31 C)
SEA SURFACE TEMPERATURES (SST), AND STRONG POLEWARD OUTFLOW, OFFSET
BY TERRAIN INTERACTION OVER KYUSHU. THE INITIAL POSITION IS PLACED
WITH MEDIUM CONFIDENCE BASED ON THE AFOREMENTIONED ANIMATED MSI AND
89GHZ GMI MICROWAVE IMAGE. THE INITIAL INTENSITY OF 35 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE LISTED AGENCY DVORAK
FIXES AND OBJECTIVE INTENSITY ESTIMATES BELOW.
INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK
CURRENT STEERING MECHANISM: A SUBTROPICAL RIDGE TO THE EAST
AGENCY DVORAK AND AUTOMATED FIXES:
PGTW: T2.5 - 35 KTS
RJTD: T2.0 - 30 KTS
RCTP: T2.5 - 35 KTS
KNES: T2.5 - 35 KTS
CIMSS ADT: 35 KTS AT 040540Z
CIMSS AIDT: 39 KTS AT 040540Z
CIMSS D-MINT: 36 KTS AT 040603Z
CIMSS D-PRINT: 28 KTS AT 040540Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
VWS: 15-20 KTS
SST: 30-31 CELSIUS
OUTFLOW: STRONG POLEWARD
OTHER FACTORS: TERRAIN INTERACTION
ANALYSIS CONFIDENCE:
INITIAL POSITION: MEDIUM
INITIAL INTENSITY: MEDIUM
INITIAL WIND RADII: LOW
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: TROPICAL STORM (TS) 21W IS FORECAST TO TRACK
NORTHEASTWARD ALONG THE NORTHWESTERN PERIPHERY OF THE SUBTROPICAL
RIDGE AND SKIRT THE SOUTHERN COAST OF JAPAN THROUGH TAU 24.
AFTERWARD, AN UPPER-LEVEL TROUGH IS EXPECTED TO DEEPEN OVER CENTRAL
HONSHU AND INTERACT WITH THE WIND FIELD OF 21W, PULLING IT
EASTWARD. THE SYSTEM IS FORECAST TO QUICKLY BECOME ABSORBED INTO
THE FORMING FRONTAL BOUNDARY ASSOCIATED WITH THE DEEP LAYER TROUGH.
AS A RESULT, EXTRATROPICAL TRANSITION IS FORECAST TO BEGIN NEAR TAU
24 AND COMPLETE NO LATER THAN TAU 48. 21W IS EXPECTED TO INTENSIFY
SLIGHTLY AS IT SKIRTS THE COAST DUE TO THE ROBUST POLEWARD OUTFLOW
AND VERY WARM SST. THE STRONGEST WINDS ARE FORECAST TO REMAIN OFF
THE COAST OF JAPAN UNTIL AFTER 21W TRACKS COMPLETELY OFFSHORE
BETWEEN TAUS 24 AND 36. AS 21W TRACKS AWAY FROM MAINLAND JAPAN, THE
WIND FIELD IS EXPECTED TO RAPIDLY EXPAND IN ASSOCIATION WITH THE
DEVELOPING MID-LATITUDE CYCLONE. SOME GLOBAL MODELS DO SHOW A
POSSIBILITY FOR 21W TO SEPARATE FROM THE FRONTAL BOUNDARY AND
REMAIN QUASI-STATIONARY NEAR THE 150E LONGITUDE BEFORE DISSIPATING,
ADDING SOME UNCERTAINTY TO THE END OF THE FORECAST.
MODEL DISCUSSION: DETERMINISTIC MODELS ARE IN GOOD AGREEMENT
THROUGH TAU 24, BUT QUICKLY DIVERGE AS INTERACTION WITH THE DEEP
LAYER TROUGH COMMENCES. THE NORTHERNMOST MODELS (AI MODELS, HAFS,
HWRF, AND EGRR) TRACK THE CIRCULATION ASSOCIATED WITH THE
MID-LATITUDE CYCLONE, DRIVING THE VORTEX TO ACCELERATE TO THE
NORTHEAST. THE SOUTHERNMOST MODELS (GFS, JGSM, AND ECMWF) TRACK THE
REMNANT VORTICITY ASSOCIATED WITH 21W AFTER IT GETS ABSORBED INTO
THE FRONTAL BOUNDARY. THE JTWC TRACK FORECAST IS PLACED WITH MEDIUM
CONFIDENCE THROUGH TAU 24 AND LOW CONFIDENCE THROUGH TAU 48.
INTENSITY GUIDANCE IS IN POOR AGREEMENT FOR THE FORECAST PERIOD.
HAFS-A AND HWRF TRACK THE MID-LATITUDE CYCLONE AND THEREFORE
RAPIDLY INTENSITY AFTER TAU 24. ON THE OTHER HAND, THE GFS AND ECMWF
WEAKEN 21W AS IT BECOMES ABSORBED INTO THE FRONTAL BOUNDARY. THE
SPLIT IN INTENSITY GUIDANCE CAUSES THERE TO BE LOW CONFIDENCE IN
THE JTWC INTENSITY FORECAST AFTER TAU 24.
FORECAST CONFIDENCE:
TRACK 00-72 HR: LOW
INTENSITY 00-72 HR: LOW//
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