Advisories

WDPN32 PGTW 300900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (ETAU) WARNING 
NR 014//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 30.8N 165.3E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 694 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: EASTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
SHALLOW, FULLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC)
ASSOCIATED WITH TROPICAL DEPRESSION (TD) 21W. THE SYSTEM, NOW
LOCATED APPROXIMATELY 180 NM SOUTHWEST OF TS 20W, PERFORMED A
SOUTHWARD LOOP IN RESPONSE TO THE FUJIWHARA EFFECT WITH TS 20W OVER
THE LAST 12 HOURS. THE BEST TRACK IS UNABLE TO DEPICT THIS LOOP DUE
TO THE 06-HOURLY TEMPORAL RESOLUTION. THE SYSTEM IS NOW MOVING
EASTWARD, DRIVEN BY THE SOUTHERN PERIPHERY OF THE CIRCULATION OF TS
20W TO THE NORTH. CONVECTIVE ACTIVITY NEAR THE CENTER IS WEAK,
LARGELY DUE TO THE HIGHLY UNFAVORABLE CONDITIONS OF COOL SEA
SURFACE TEMPERATURES (SST), DRY AIR, AND CONVERGENCE ALOFT AS AN
UPPER-LEVEL LOW SITS OVERHEAD. THE INITIAL POSITION IS ASSESSED
WITH HIGH CONFIDENCE. THE INITIAL INTENSITY IS SET AT 30 KTS WITH
MEDIUM CONFIDENCE BASED ON A BLEND OF THE T2.0-2.5 FIXES FROM PGTW
AND RJTD AND THE RECENT OBJECTIVE ESTIMATES FROM CIMSS RANGING FROM
25 KTS TO 33 KTS.

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

CURRENT STEERING MECHANISM: BINARY INTERACTION WITH TROPICAL STORM
(TS) 20W

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.5 - 35 KTS
   CIMSS SATCON: 45 KTS AT 300340Z
   CIMSS ADT: 25 KTS AT 300610Z
   CIMSS AIDT: 33 KTS AT 300610Z
   CIMSS D-MINT: 33 KTS AT 300518Z
   CIMSS D-PRINT: 31 KTS AT 300610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY UNFAVORABLE
   VWS: 15-20 KTS
   SST: 25-27 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: DRY AIR ENVIRONMENT

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST TRACK HAS SHIFTED
EASTWARD TO ACCOUNT FOR THE FUJIWHARA EFFECT.

FORECAST DISCUSSION: TD 21W WILL TURN POLEWARD OVER THE NEXT 24
HOURS AS THE CIRCULATION OF TS 20W PULLS AWAY. ONCE THIS OCCURS, AN
EXTENSIVE RIDGE OVER THE CENTRAL NORTH PACIFIC WILL CONTINUE TO
GUIDE THE SYSTEM NORTHWARD FOR THE REMAINDER OF THE FORECAST
PERIOD. THE 25-35 KTS OF SOUTHWESTERLY VERTICAL WIND SHEAR HAS
EASED SIGNIFICANTLY. HOWEVER, THE ENVIRONMENT WILL STILL BE
UNFAVORABLE, WITH INSUFFICIENTLY WARM SST, SURROUNDING DRY AIR, AND
THE SUPPRESSION OF OUTFLOW ALOFT. THE SPORADIC CONVECTION MAY BE
ENOUGH TO KEEP THE CIRCULATION SPINNING AT 30 KTS AS IT GRADUALLY
UNDERGOES SUBTROPICAL TRANSITION AND BECOMES A SUBTROPICAL
DEPRESSION BY TAU 48. SOME UNCERTAINTY NOW EXISTS DUE TO THE
PROXIMITY OF TS 20W. THERE IS A POSSIBILITY THAT TS 20W WILL MERGE
WITH TD 21W. UNDER THIS SCENARIO, OUTRIGHT DISSIPATION WOULD OCCUR
WITHIN 24 HOURS.

MODEL DISCUSSION: CROSS-TRACK SPREAD IS LARGE AMONG THE TRACKER
GUIDANCE PRIMARILY DUE TO UNCERTAINTIES ASSOCIATED WITH THE
INTERACTION WITH TS 20W. THE JTWC FORECAST TRACK HAS SHIFTED
EASTWARD, LEANING CLOSEST TO THE GFS, HAFS-A, AND NAVGEM SOLUTIONS,
WHICH MAKE UP THE EASTERN SIDE OF THE GUIDANCE ENVELOPE, THROUGH
TAU 12 TO ACCOUNT FOR THE RECENT STORM MOTION. THE TRACK THEN
BLENDS BACK TOWARD THE AI CONSENSUS BY TAU 48. THE INTENSITY AIDS
GENERALLY DEPICT A STEADY TREND THROUGH TAU 48, AND THE JTWC
REFLECTS THIS TREND WITH MEDIUM CONFIDENCE.

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