WDPN31 PGTW 050900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (PEIPAH) WARNING NR 
008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 34.9N 139.3E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 31 NM SOUTHWEST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 30 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 21W (PEIPAH) WITH PERSISTENT DEEP CONVECTION OBSCURING THE
LOW-LEVEL CIRCULATION CENTER (LLCC). A 050523Z GMI 89 GHZ MICROWAVE
IMAGE SHOWS THE VIGOROUS BURSTS OF CONVECTION NEAR THE CENTER WITH
LOW-LEVEL CLOUDS RUNNING NEARLY PARALLEL TO THE COAST. THE WIND
FIELD REMAINS HIGHLY ASYMMETRIC WITH THE STRONGEST WINDS WITHIN THE
SOUTHERN SEMICIRCLE. A MAJOR SHORTWAVE TROUGH HAS STARTED TO DIG
OVER THE EASTERN SEA OF JAPAN WHICH HAS INCREASED WESTERLY VERTICAL
WIND SHEAR. OUTFLOW IN BOTH THE POLEWARD AND EQUATORWARD DIRECTIONS
REMAIN VERY STRONG, APPARENT BY THE CIRRUS FILAMENTS EXPANDING
OUTWARD FROM THE SYSTEM. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE WIND DIRECTION SHIFT AT THE STATIONS NEAR
THE IZU PENINSULA, SHIFTING FROM SSW TO NNE NEAR 0500Z. THE INITIAL
INTENSITY OF 45 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON
MULTIPLE SURFACE OBSERVATIONS TO THE SOUTH REPORTING AROUND 40 KTS.
ADDITIONALLY, AGENCY DVORAK FIXES FROM PGTW AND KNES SUPPORT THE
INITIAL INTENSITY ASSESSMENT.

INITIAL WIND RADII BASIS: PERSISTENCE FROM 042346Z METOP-B ASCAT
SCATTEROMETER DATA THAT IS SLIGHTLY MODIFIED BY NEARBY SURFACE
OBSERVATIONS

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A SUBTROPICAL
RIDGE CENTERED TO THE SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 46 KTS AT 050330Z
   CIMSS D-PRINT: 37 KTS AT 050600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD
   OTHER FACTORS: TERRAIN INTERACTION

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: 21W IS FORECAST TO TRACK GENERALLY EASTWARD
THROUGH TAU 24 UNDER THE INFLUENCE OF BOTH THE STEERING RIDGE TO
THE SOUTH AND THE EASTWARD PROPAGATING MAJOR SHORTWAVE TROUGH. 21W
IS ANALYZED TO HAVE STARTED EXTRATROPICAL TRANSITION (ETT),
APPARENT BY THE INCREASING INTERACTION WITH THE UPPER-LEVEL FLOW
AND INCREASING VERTICAL WIND SHEAR. THE SYSTEM IS NOW ASSUMING THE
TRAITS OF BOTH TROPICAL AND EXTRATROPICAL CYCLONES. AS THE TROUGH
TO THE NORTH CONTINUES TO DEEPEN, THERMAL ADVECTION OVER THE SYSTEM
WILL INCREASE, CAUSING 21W TO TRANSITION INTO AN EXPANSIVE
STORM-FORCE EXTRATROPICAL CYCLONE. THIS ETT PROCESS IS FORECAST TO
COMPLETE NEAR TAU 24. BAROCLINIC FORCING WILL RESULT IN
INTENSIFICATION OF THE WIND FIELD AS THE SYSTEM TRACKS AWAY FROM
THE EASTERN COAST OF HONSHU, UP TO AROUND 55 KTS. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE HAS TIGHTENED OVER
THE PAST 6-12 HOURS. CROSS TRACK SPREAD AT TAU 24 IS 150 NM, AND
ALONG-TRACK SPREAD HAS SIGNIFICANTLY DECREASED COMPARED TO PREVIOUS
MODEL RUNS. AS A RESULT, THE JTWC TRACK FORECAST IS PLACED CLOSE TO
THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE. INTENSITY GUIDANCE
IS IN MODERATE AGREEMENT WITH INTENSITIES RANGING FROM 50-65 KTS AT
TAU 24. HAFS-A CONTINUES TO BE THE MOST AGGRESSIVE MODEL. THE JTWC
INTENSITY FORECAST IS PLACED WITH MEDIUM CONFIDENCE DUE TO THE
SPREAD IN GUIDANCE AND UNCERTAINTIES REGARDING THE STATE OF THE
VORTEX AS IT COMPLETES ETT. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   INTENSITY 00-72 HR: MEDIUM//
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