WDPN31 PGTW 031500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.0N 131.6E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 648 NM SOUTHWEST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL DEPRESSION (TD) 21W IS LOCATED WITHIN A WIDE SWATH OF UPPER-
LEVEL SOUTHERLY WINDS BETWEEN AN UPPER-LOW TO THE SOUTHWEST AND AN 
ANTICYCLONE NEAR 30N 135E. AS A RESULT, THERE IS A TIGHT GRADIENT OF 
LOW TO HIGH (15 TO 25 KNOTS) VERTICAL WIND SHEAR STRADDLING THE 
SYSTEM. HOWEVER, POLEWARD OUTFLOW REMAINS ROBUST, WITH EXPANDING 
EQUATORWARD OUTFLOW OVER THE SOUTHERN SEMICIRCLE. AS INDICATED IN 
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY, THIS HAS ALLOWED 
THE SYSTEM TO SLOWLY CONSOLIDATE, WITH DEEP CONVECTIVE BANDING 
WRAPPING FROM THE EASTERN SEMICIRCLE INTO THE NORTHWEST QUADRANT. EIR 
SHOWS A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER ALONG THE 
SOUTHERN EDGE OF THE CORE CONVECTION, WHICH SUPPORTS THE INITIAL 
POSITION WITH MEDIUM CONFIDENCE. THE INITIAL INTENSITY OF 30 KTS IS 
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK ESTIMATES 
AND RECENT ASCAT DATA SHOWING 25-30 KNOT WINDS OVER THE EASTERN 
SEMICIRCLE.

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

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR)
ENTRENCHED TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TD 21W IS FORECAST TO TRACK NORTHWARD THROUGH
TAU 24 ALONG THE WESTERN PERIPHERY OF THE STR, WITH STEADY 
INTENSIFICATION TO A PEAK INTENSITY OF 40 KNOTS AT TAU 24. AFTER TAU 
24, THE SYSTEM WILL RECURVE AROUND THE NORTHWESTERN PERIPHERY OF THE 
STR ON A GENERALLY EAST-NORTHEASTWARD TRAJECTORY. TD 21W IS EXPECTED 
TO WEAKEN SLIGHTLY AS IT TRACKS ACROSS JAPAN BUT IS FORECAST TO 
MAINTAIN TROPICAL STORM STRENGTH WINDS OVER THE EASTERN AND 
SOUTHEASTERN QUADRANTS. NEAR TAU 48, THE SYSTEM WILL COMMENCE EXTRA-
TROPICAL TRANSITION (ETT) AS A MAJOR SHORTWAVE TROUGH DIGS OVER 
CENTRAL JAPAN, WITH JET STRENGTH WINDS OVER CENTRAL HONSHU
SUPPORTING A DEVELOPING FRONT. GLOBAL MODELS INDICATE AN INCREASINGLY 
COMPLEX SYNOPTIC PATTERN DEVELOPING NEAR TAU 48, WITH A DEEP LOW 
PRESSURE AREA FORMING NORTHEAST OF TOKYO AND EXPANSIVE REGION OF GALE 
FORCE WINDS EAST OF JAPAN. TD 21W IS EXPECTED TO GET ABSORBED INTO 
THIS MIDLATITUDE LOW OR ALONG THE FRONTAL BOUNDARY BY TAU 72, 
COMPLETING ETT. THE 34-KNOT WIND RADII EXPANDS RAPIDLY AT TAU 72 DUE 
TO THE FORMATION OF THE STRONG MIDLATITUDE LOW AND FRONTAL SYSTEM.          

MODEL DISCUSSION: DETERMINISTIC GUIDANCE IS IN GOOD AGREEMENT THROUGH 
TAU 48 BUT DIVERGES AFTER TAU 48 DUE TO THE INCREASINGLY COMPLEX, 
RAPIDLY EVOLVING FRONTAL SYSTEM AND MIDLATITUDE LOW. SOME OF THE 
MESOSCALE INTENSITY GUIDANCE (COAMPS-TC GFS, HAFS-A AND HWRF) APPEAR 
TO ERRONEOUSLY JUMP TO THE RAPIDLY DEEPENING MIDLATITUDE LOW, WITH 
PEAK INTENSITY VALUES RANGING FROM 60-90 KNOTS AT TAU 60 OFF THE 
COAST OF JAPAN. THE JTWC INTENSITY FORECAST IS SIGNIFICANTLY LOWER 
BASED ON THE EXPECTATION OF THE SYSTEM GETTING ABSORBED INTO THE 
MIDLATITUDE LOW AND FRONTAL SYSTEM. THE 030600Z ECMWF ENSEMBLE (EPS) 
AND GFS ENSEMBLE (GEFS) SHOW GOOD AGREEMENT ON A NORTHWARD TRACK OVER 
SOUTHERN KYUSHU THEN A SHARP EASTWARD TRACK ACROSS CENTRAL HONSHU. IN 
GENERAL, THE SPREAD OF SOLUTIONS EXTENDS FROM CENTRAL HONSHU TO SOUTH 
OF THE TOKYO REGION.

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