WDPN31 PGTW 042100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (PEIPAH) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 33.1N 132.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 80 NM SOUTH-SOUTHEAST OF IWAKUNI, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS DEEP
CONVECTION COMPLETELY OBSCURING THE LOW-LEVEL CIRCULATION CENTER
(LLCC) OF TROPICAL STORM (TS) 21W. DRY AIR IS ENTRAINING INTO THE
SYSTEM VIA A DRY SLOT ALONG THE SOUTHERN PERIPHERY OF THE
CIRCULATION. WATER VAPOR IMAGERY DEPICTS STRONG POLEWARD AND
EQUATORWARD OUTFLOW. AN UPPER-LEVEL TROUGH UPSTREAM OF TS 21W IS
ENHANCING THE POLEWARD OUTFLOW STRONGLY. THE INITIAL POSITION IS
PLACED WITH HIGH CONFIDENCE BASED ON A 041755Z GCOM W-1 AMSR2 IMAGE
REVEALING THE ASYMMETRIC CONVECTIVE FIELD POLEWARD AND EASTWARD OF
CENTER DUE TO STRONG 20-25KT WESTERLY VERTICAL WIND SHEAR. THE
INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON LOCAL OBSERVATIONS SOUTHEAST OF THE CIRCULATION CENTER.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: A SUBTROPICAL RIDGE TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T2.0 - 30 KTS
   CIMSS SATCON: 47 KTS AT 041655Z
   CIMSS ADT: 43 KTS AT 041730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: OVER LAND
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TS 21W WILL CONTINUE ON AN EAST-NORTHEAST
CURVING TRACK FOR THE NEXT 24 HOURS. BY TAU 24, THE SYSTEM WILL
BECOME ABSORBED INTO A LARGER MIDLATITUDE CYCLONE DRIVEN BY AN
APPROACHING SHORTWAVE TROUGH TO THE NORTH. FOR THE NEST 12 HOURS,
TS 21W WILL STRUGGLE TO INTENSIFY AS THE SYSTEM INTERMITTENTLY
INTERACTS WITH TOPOGRAPHY AND MODERATE TO HIGH VERTICAL WIND SHEAR
ALONG THE SOUTHERN COAST OF JAPAN. AS THE SYSTEM BECOMES BAROCLINIC
AT TAU 24, THE STORM WILL BEGIN TO INTENSIFY GRADUALLY WITH
THE STRONGEST WINDS ALONG THE SOUTHERN HALF OF THE WIND FIELD. BY
THE END OF THE FORECAST PERIOD, TS 21W WILL TRACK WESTWARD AT A
PEAK INTENSITY OF 45KTS, BECOMING A FULLY EXTRATROPICAL SYSTEM.


MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THAT TS 21W WILL CURVE EASTWARD FOR THE REMAINDER OF THE FORECAST
PERIOD. THERE IS MINIMAL CROSS-TRACK SPREAD IN THE TRACK GUIDANCE
AND THE TRACK SPEED IS CONSISTENT. THE CROSS-TRACK SPREAD IS 70NM
AT TAU 24 AND OPENS TO 170NM AT TAU 36. THERE IS MOSTLY GOOD
AGREEMENT IN THE INTENSITY GUIDANCE WITH PEAK INTENSITIES BETWEEN
40-50KTS, AND AN EXTRANEOUS PEAK FOR HAFS-A OF 70KTS.

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