WDPN31 PGTW 062100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.9N 145.7E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 66 NM NORTH OF AGRIHAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS DEEP
CONVECTION PERSISTING ALONG THE EASTERN AND NORTHERN PERIPHERIES OF
A COMPACT LOW-LEVEL CIRCULATION CENTER (LLCC) EVIDENT IN A 061621Z
AMSR2 PASS. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON 37 GHZ IMAGERY FROM THE 061621Z AMSR2 PASS, A 061800Z CIRA
PROXYVIS IMAGE, AND THE 061730Z PGTW SATELLITE FIX POSITION. THE
INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED
ON A CONSENSUS OF SUBJECTIVE DVORAK AND OBJECTIVE INTENSITY FIX
ESTIMATES.

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

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS SATCON: 35 KTS AT 061601Z
   CIMSS ADT: 34 KTS AT 061740Z
   CIMSS DPRINT: 28 KTS AT 061800Z

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   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 TRACK
NORTHEASTWARD UNDER THE CONTINUED INFLUENCE OF SUBTROPICAL RIDGING
TO THE SOUTH AND EAST DURING THE FORECAST PERIOD. BY TAU 72, TD 21W
WILL BEGIN TO ACCELERATE POLEWARD AHEAD OF A DEVELOPING MID-LATITUDE 
TROUGH APPROACHING FROM THE NORTHWEST. THE SYSTEM WILL INTERACT WITH 
THIS TROUGH AND START THE EXTRATROPICAL TRANSITION PROCESS BY TAU 96, 
BECOMING FULLY EMBEDDED IN THE MID-LATITUDE FLOW PATTERN BY TAU 120. TD 
21W IS MOVING INTO AN AREA OF PERSISTENT, MODERATE TO HIGH  
SOUTHWESTERLY VERTICAL WIND SHEAR (VWS), WHICH WILL LIMIT 
INTENSIFICATION TO A SLOW RATE THROUGH THE FIRST 48 HOURS OF THE 
FORECAST PERIOD DESPITE PASSAGE OVER VERY WARM WATER AND SUPPORTIVE 
DIFFLUENCE ALOFT. AFTER TAU 48, IMPROVING POLEWARD OUTFLOW THROUGH TAU 
72, FOLLOWED BY FAVORABLE BAROCLINIC SUPPORT DURING THE SUBSEQUENT 
EXTRATROPICAL TRANSITION, WILL RESULT IN STEADY INTENSIFICATION AND 
GROWTH OF THE WIND FIELD. THE SYSTEM WILL BE A STRONG EXTRATROPICAL LOW 
BY TAU 120.

MODEL DISCUSSION: NUMERICAL MODEL TRACK FORECAST GUIDANCE IS IN
GOOD AGREEMENT, WITH SPREAD INCREASING A BIT AFTER TAU 72 DUE TO
DIFFERING REPRESENTATIONS OF THE STEERING FLOW PATTERN AND INTERACTION 
WITH THE DEVELOPING MID-LATITUDE TROUGH DURING THE EXTENDED PERIOD. 
INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT THROUGH THE FORECAST 
PERIOD, DEPICTING SLOW INTENSIFICATION IN THE NEAR TO MEDIUM-TERM 
LIMITED BY PERSISTENT VWS, AND INTENSIFICATION DURING EXTRATROPICAL 
TRANSITION IN THE EXTENDED FORECAST PERIOD.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: HIGH
   INTENSITY 72-120 HR: HIGH//
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