WDPN31 PGTW 050300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (PEIPAH) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 33.9N 135.5E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 171 NM EAST OF IWAKUNI, JAPAN
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 24 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A DEEP
CONVECTIVE STRUCTURE WHICH IS HIGHLY SHEARED BY 25-30 KTS OF
VERTICAL WIND SHEAR IMPOSED BY AN UPSTREAM SHORTWAVE TROUGH TO THE
NORTHWEST. CLEAR, DRY AIR IS ENTRAINED IN A VISIBLE DRY SLOT IN THE
SOUTHERN PERIPHERY OF THE SYSTEM. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON A LOW-LEVEL CLOUD BANDING EVIDENT ON
RADAR IMAGERY. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH
HIGH CONFIDENCE BASED ON LOCAL OBSERVATIONS AND 052348Z METOP-B
ASCAT WIND VECTOR IMAGE HIGHLIGHTING THE ASYMMETRIC WIND FIELD
OFFSHORE WITH ENHANCED WINDS TO THE SOUTH AND EAST OF THE
CIRCULATION CENTER.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: A SUBTROPICAL RIDGE TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T2.0 - 30 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: OVER LAND
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   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 WILL CONTINUE ON A
EAST-NORTHEASTWARD TRACK AS THE SYSTEM CONTINUES TO ROUND THE
NORTHERN PERIPHERY OF THE SUBTROPICAL RIDGE. THE SYSTEM HAS BEGUN
EXTRATROPICAL TRANSITION, AND IS ASSUMING TRAITS OF BOTH TROPICAL
AND EXTRATROPICAL CYCLONES. THE APPROACHING UPPER-LEVEL TROUGH WILL
INCREASE THERMAL ADVECTION GRADUALLY OVER THE NEXT 24 HOURS.
BAROCLINIC FORCING WILL RESULT IN INTENSIFICATION TO A PEAK OF
50KTS BY TAU 24 WHEN TS 21W BECOMES STRONGLY BAROCLINIC. THE
APPROACHING UPPER-LEVEL TROUGH WILL STRENGTHEN THERMAL ADVECTION
GRADUALLY OVER THE NEXT 24 HOURS. THE SYNOPTIC ENVIRONMENT IS
COMPLEX BY THE END OF THE FORECAST, AND THE VORTICITY OF THE
TRANSITIONING TS 21W WILL BECOME EMBEDDED IN A LARGER FRONTAL
REGION OF A DEVELOPING MIDLATITUDE CYCLONE. THERE IS UNCERTAINTY
BETWEEN MODEL GUIDANCE WHETHER THE FRONT WILL ELONGATE AND BREAK
INTO TWO DISTINCT CIRCULATIONS, OR REMAIN ONE LARGE MIDLATITUDE
SYSTEM BEYOND THE FORECAST PERIOD, BUT THE CURRENT FORECAST ENDS AS
THE SYSTEM BECOMES FULLY EXTRATROPICAL AND EMBEDDED WITHIN THE
MIDLATITUDE FRONT.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THAT TS 21W WILL TRACK EASTWARD OVER THE NEXT 24 HOURS. SOME
VORTICITY TRACKERS, SUCH AS NAVGEM, GFS, AND GALWEM, TRACK THE
CIRCULATION NORTHWARD WHERE THE CIRCULATION WILL BECOME FULLY
ABSORBED INTO THE MIDLATITUDE CYCLONE. ON THE OTHER HAND, THE
CIRCULATION MAY CURVE EAST-SOUTHEASTWARD AND BREAK OFF OF THE
ELONGATED FRONTAL REGION THAT THE SYSTEM BECOMES ABSORBED INTO. BY
THE END OF THE FORECAST PERIOD, THERE IS A 110NM CROSS-TRACK
SPREAD. THE INTENSITY GUIDANCE IS ALSO SPREAD, WITH FINAL
INTENSITIES BETWEEN 35-65KTS. HOWEVER, THE CURRENT INTENSITY
FORECAST REFLECTS THE CURRENT ENVIRONMENTAL FEATURES FORCING
INTENSIFICATION.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: LOW//
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