WDPN31 PGTW 040300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (PEIPAH) WARNING NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 29.7N 131.1E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 216 NM SOUTH-SOUTHEAST OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 16 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 DEEP CONVECTION
DISLOCATED TO THE NORTH BY 15-20 KTS OF SOUTHERLY VERTICAL WIND
SHEAR. WATER VAPOR IMAGERY REVEALS WEAK RADIAL OUTFLOW ALOFT. THE
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON ANIMATED
VISIBLE IMAGERY REVEALING THE PARTIALLY EXPOSED LLCC AND A 040009Z
METOP-B ASCAT IMAGE REVEALING THE LOW-LEVEL CIRCULATION CENTER. THE
INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED
ON THE AFOREMENTIONED SCATTEROMETRY WITH AN ASYMMETRIC WIND FIELD
WITH THE STRONGEST WINDS ON THE EASTERN PERIPHERY AND WEAKER WINDS
ALONG THE WESTERN SIDE.

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: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   CIMSS ADT: 33 KTS AT 032030Z

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

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 CURVE
NORTHEASTWARD ROUNDING THE PERIPHERY OF THE SUBTROPICAL RIDGE
LOCATED TO THE EAST FOR THE NEXT 24 HOURS. AFTER TAU 24, THE TRACK
WILL CURVE EAST-NORTHEASTWARD FOR THE REMAINDER OF THE FORECAST
PERIOD. THROUGH TAU 36, THE SYSTEM WILL
SLIGHTLY DEVELOP IN A MARGINALLY FAVORABLE ENVIRONMENT WITH
MODERATE VERTICAL WIND SHEAR AND WARM SEA SURFACE TEMPERATURES. THE
REMAINING VORTICITY OF TS 21W WILL WEAKEN DUE TO TOPOGRAPHIC
INTERACTION, AND BECOME EMBEDDED INTO A FRONTAL BOUNDARY THAT
DEVELOPS WITH A MAJOR SHORTWAVE TROUGH DEEPENS OVER CENTRAL JAPAN.
THE UPPER-LEVEL TROUGH IS ANTICIPATED TO CONTINUE TO DEEPEN
THROUGH THE END OF THE FORECAST INTO A STRONG MIDLATITUDE CYCLONE,
AND BY TAU 48, TS 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 ROUND THE NORTHERN PERIPHERY OF THE SUBTROPICAL
RIDGE TO THE EAST. HOWEVER, AFTER TAU 36, THE TRACK SPREAD
INCREASES AS THE MODEL TRACKERS STRUGGLE TO FOLLOW THE VORTICITY
MAXIMUM. SOME TRACKERS MOVE NORTHWARD, ALMOST DIRECTLY INTO THE
CENTER OF THE DEEP LOW, WHILE THE OTHER TRACKERS FOLLOW THE TAIL
END OF THE FRONTAL REGION. THE INTENSITY GUIDANCE FOLLOWS THE TRACK
GUIDANCE, WITH PEAK INTENSITIES REACHING 70KTS AT TAU 48 FOR TRACK
MEMBERS WHICH FOLLOW THE MIDLATITUDE CYCLONE. OTHER MEMBERS WHICH
DEPICT ABSORPTION INTO THE FRONT REMAIN MUCH LOWER BETWEEN
35-45KTS. 

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