WDPN31 PGTW 041500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (PEIPAH) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 32.3N 132.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 424 NM WEST-SOUTHWEST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPLETED OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC) WITH
PERSISTENT CONVECTION OVER THE CENTER. ADDITIONALLY, A DEFINED
CONVECTIVE BAND IS WRAPPING AROUND THE EASTERN SEMICIRCLE WITH A
DRY SLOT TO ITS IMMEDIATE WEST. A 041126Z METOP-B ASCAT IMAGE
REVEALS AN ASYMMETRIC WIND FIELD WITH 25-30 KT WINDS CONFINED TO
THE SOUTHEASTERN QUADRANT. THE ENVIRONMENT IS ASSESSED TO BE
MARGINAL, 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 ALOFT, OFFSET BY TERRAIN INTERACTION
WITH SOUTHERN JAPAN AND DRY AIR ENTRAINMENT. THE INITIAL POSITION
IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE 041126Z METOP-B ASCAT
IMAGE HINDERED BY PROXIMITY TO LAND. THE INITIAL INTENSITY OF 35
KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED
METOP-B ASCAT IMAGE, AND THE LISTED AGENCY DVORAK FIXES AND
OBJECTIVE INTENSITY ESTIMATES BELOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF THE
SUBTROPICAL RIDGE TO THE EAST OF THE SYSTEM

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T3.0 - 45 KTS
   KNES: T2.5 - 35 KTS
   CIMSS ADT: 34 KTS AT 041200Z
   CIMSS AIDT: 40 KTS AT 041200Z
   CIMSS D-PRINT: 36 KTS AT 041200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: TERRAIN INTERACTION AND DRY AIR ENTRAINMENT

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

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
CONTINUE ITS NORTHEASTWARD TRACK ALONG THE NORTHWESTERN PERIPHERY OF
THE SUBTROPICAL RIDGE. 21W WILL TRACK ALONG THE SOUTHERN COAST OF
JAPAN, SKIRTING THE COAST UNTIL IT EMERGES OVER WATER AT TAU 24.
AFTER TAU 24, A DEEP UPPER-LEVEL TROUGH IS EXPECTED TO QUICKLY
ABSORB 21W, COMPLETING EXTRATROPICAL TRANSITION (ETT) NO LATER THAN
TAU 36. UP UNTIL ETT, 21W WILL MARGINALLY INTENSIFY DUE TO ROBUST
POLEWARD OUTFLOW AND VERY WARM SSTS, HINDERED BY LAND INTERACTION
AND DRY AIR ENTRAINMENT. THE STRONGEST WINDS ARE FORECAST TO REMAIN
OFF THE COAST OF JAPAN UNTIL AFTER 21W TRACKS COMPLETELY OFFSHORE
AFTER TAU 24. AS THE SYSTEM UNDERGOES ETT, THE WIND FIELD WILL
EXPAND AND INTENSIFY TO 45 KTS DUE TO BAROCLINIC FORCING. SOME
DETERMINISTIC MODELS INDICATE THE POSSIBILITY OF 21W DETACHING
FROM THE FRONTAL BOUNDARY AFTER TAU 36 AND REMAINING
QUASI-STATIONARY NEAR THE 155E LONGITUDE BEFORE DISSIPATING. AS A
RESULT, THERE IS SOME UNCERTAINTY AT THE END OF THE JTWC TRACK
FORECAST.

MODEL DISCUSSION: DETERMINISTIC MODELS ARE IN MODERATE AGREEMENT
THROUGH TAU 24, BUT QUICKLY DIVERGE THROUGH TAU 36 AS TS 21W IS
INFLUENCED BY THE UPPER-LEVEL TROUGH. AT TAU 36, THERE IS A 490NM
CROSS-TRACK SPREAD WITHIN THE MODEL GUIDANCE. THE NORTHERNMOST
MODELS (AI MODELS AND EGRR) TRACK THE CIRCULATION ASSOCIATED WITH
THE DEVELOPING 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 IS PLACED
WITH LOW CONFIDENCE DUE TO THE LARGE MODEL SPREAD AND COMPLEX
SYNOPTIC SITUATION THROUGH TAU 36. INTENSITY GUIDANCE IS IN POOR
AGREEMENT WITH HAFS-A AND HWRF BOTH INTENSIFYING THE VORTEX AT A
MUCH QUICKER PACE THAN THE GLOBAL MODELS AND COAMPS-TC. THE
DEVELOPMENT OF THE MID-LATITUDE CYCLONE, AND ABSORPTION OF 21W
CAUSES THE JTWC INTENSITY FORECAST TO ALSO BE PLACED WITH LOW
CONFIDENCE. 

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