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// NNNN