WDPN31 PGTW 032100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (PEIPAH) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 28.1N 131.1E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 376 NM SOUTH OF IWAKUNI, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
LOW-LEVEL CIRCULATION CENTER (LLCC) WHICH HAS BEEN EXPOSED BY
SOUTHERLY 15-20KT VERTICAL WIND SHEAR (VWS). THE ENHANCED SHEAR HAS
PREVENTED THE UPPER-LEVEL ENVIRONMENT FROM DEVELOPING STRONGLY,
RESULTING IN RAGGED AND DISORGANIZED CONVECTIVE BANDING.
UPPER-LEVEL SOUTHERLY FLOW FROM A TROPICAL UPPER TROPOSPHERIC
TROUGH (TUTT) CELL TO THE WEST. PARTIAL RADAR IMAGERY FROM JAPAN
REVEALS THE POLEWARD TILT, HIGHLIGHTING THE CIRCULATION CENTER
SLIGHTLY NORTH OF THE POSITION AT THE SURFACE. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED RADAR
AND EIR IMAGERY. THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH
LOW CONFIDENCE BASED ON THE AGENCY AND OBJECTIVE FIXES LISTED
BELOW.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: A SUBTROPICAL RIDGE TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   CIMSS ADT: 30 KTS AT 031430Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: LOW
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 21W WILL CONTINUE
NORTHWARD FOR THE NEXT 12 HOURS WHILE CROSSING THE AXIS OF THE
SUBTROPICAL RIDGE TO THE EAST.  BETWEEN TAU 12-36, TD 21W WILL
CURVE SLOWLY ALONG A NORTHEASTWARD TRACK BEFORE MOVING IN THE
EASTWARD DIRECTION AFTER TAU 36. UNTIL TAU 36, THE SYSTEM WILL
SLIGHTLY DEVELOP IN A MARGINALLY FAVORABLE ENVIRONMENT WITH
MODERATE VERTICAL WIND SHEAR AND WARM SEA SURFACE TEMPERATURES. THE
WIND FIELD WILL REMAIN ASYMMETRIC AND STRONGER ON THE EASTERN
SEMICIRCLE DUE TO ENHANCED MONSOONAL FLOW AND SLIGHT TOPOGRAPHIC
INTERACTION STARTING AT TAU 24. THE VORTICITY OF TD 21W WILL BECOME
ABSORBED INTO A DEVELOPING FRONTAL BOUNDARY BETWEEN TAU 36-48 AS A
MAJOR SHORTWAVE TROUGH DEEPENS OVER CENTRAL JAPAN. THE TROUGH IS
ANTICIPATED TO CONTINUE TO DEEPEN THROUGH THE END OF THE FORECAST
INTO A STRONG MIDLATITUDE CYCLONE, AND BY TAU 72, TD 21W WILL NO
LONGER BE TRACKABLE AS THE SYSTEM BECOMES ABSORBED INTO THE DEEP
LOW WITH ASSOCIATED GALE FORCE WINDS.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THAT TS 21W WILL CURVE ALONG THE NORTHERN PERIPHERY OF THE
SUBTROPICAL RIDGE TO THE EAST THROUGH TAU 36. THE CROSS-TRACK
SPREAD AT TAU 36 IS 86NM. HOWEVER, AS THE MIDLATITUDE CYCLONE
DEVELOPS TO THE EAST, THE VORTEX TRACKERS SEEMINGLY DO NOT FOLLOW
THE INITIAL VORTICITY OF TS 21W, BUT INSTEAD FOLLOW THE TRACK OF
THE DEVELOPING LOW. THEREFORE, THE TRACK FORECAST IS MADE WITH LESS
CONFIDENCE AFTER TAU 48 BECAUSE OF THE ABSORPTION OF THE VORTICITY
MAXIMUM INTO THE DEVELOPING FRONT. THE INTENSITY GUIDANCE IS ALSO
SPREAD AND ASSOCIATED WITH THE DEEP LOW. THE CURRENT INTENSITY
FORECAST IS REPRESENTATIVE OF THE GALE FORCE WINDS ASSOCIATED WITH
THE DEVELOPING FRONT THAT ABSORBS TD 21W AFTER TAU 36, BUT THE
INTENSITY GUIDANCE FOLLOWS THE INTENSITY TRACKERS, ASSESSING
POTENTIAL PEAK INTENSITIES OF 50-70KTS (COAMPS-TC, HWRF, HAFS-A,
GFS). 

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