WDPN32 PGTW 301500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (ETAU) WARNING NR 015// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 31.1N 166.0E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 709 NM NORTH OF WAKE ISLAND MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 07 KTS SIGNIFICANT WAVE HEIGHT: 10 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A COMPACT, PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH MEAGER CONVECTIVE ACTIVITY FLARING ALONG THE SOUTHEAST PERIPHERY. THIS TINY SURFACE CIRCULATION WAS CAPTURED BY A 300941Z ASCAT-C PASS, WHICH MEASURED WINDS NEAR 30 KTS IN THE SOUTHEAST QUADRANT. AN UPPER-LEVEL LOW PRESENTLY SITS OVERHEAD, SEVERELY RESTRICTING OUTFLOW ALOFT. THE SYSTEM IS ALSO TRAVERSING MARGINALLY WARM SEA SURFACE TEMPERATURES (SST) OF 26-27 DEGREES CELSIUS AND IS SURROUNDED BY DRY AIR. THESE ENVIRONMENTAL FACTORS ARE UNFAVORABLE FOR TROPICAL CYCLONE DEVELOPMENT. CONSEQUENTLY, THE CONVECTIVE ACTIVITY HAS BEEN MINIMAL, LEAVING THE LLCC INCREASINGLY FRAGILE. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC IN THE SHORTWAVE-INFRARED IMAGERY. THE INITIAL INTENSITY HAS BEEN HELD AT 30 KTS WITH HIGH CONFIDENCE BASED ON THE SCATTEROMETER DATA, SUPPORTED BY A BLEND OF AGENCY SUBJECTIVE DVORAK FIXES AND OBJECTIVE ESTIMATES FROM CIMSS. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: BINARY INTERACTION WITH TROPICAL STORM (TS) 20W AND AN EXTENSIVE RIDGE POSITIONED OVER THE CENTRAL NORTH PACIFIC AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS RJTD: T2.5 - 35 KTS RCTP: T1.5 - 25 KTS CIMSS ADT: 29 KTS AT 301210Z CIMSS AIDT: 25 KTS AT 301210Z CIMSS D-MINT: 35 KTS AT 301038Z CIMSS D-PRINT: 30 KTS AT 301210Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE VWS: 10-15 KTS SST: 26-27 CELSIUS OUTFLOW: WEAK POLEWARD OTHER FACTORS: UPPER-LEVEL LOW OVERHEAD AND SURROUNDING DRY AIR 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: HAVING COMPLETED A 180-DEGREE SOUTHWARD LOOP INDUCED BY THE FUJIWHARA EFFECT, TD 21W HAS BEGUN TO TURN POLEWARD AS ITS DISTANCE FROM TS 20W INCREASES. THIS POLEWARD MOTION WILL NOW BE DRIVEN BY THE STEERING INFLUENCE OF A LARGE ANTICYCLONE POSITIONED OVER THE CENTRAL NORTH PACIFIC. ALTHOUGH THE LLCC IS UNRAVELING, THE FORECAST HOLDS THE INTENSITY AT 30 KTS FOR THE NEXT 36 HOURS. THE CIRCULATION WILL BECOME EMBEDDED IN 25-30 KTS OF SOUTHERLY LOW-LEVEL FLOW. THE EVENTUAL DEMISE OF THE TROPICAL SYSTEM WILL BE DRIVEN BY TWO COMPETING, THOUGH NOT MUTUALLY EXCLUSIVE, FACTORS. FIRST, THE POLEWARD TRACK WILL BRING THE SYSTEM OVER PROGRESSIVELY COOLER SST AND INTO A MORE BAROCLINIC ENVIRONMENT, CAUSING IT TO GRADUALLY ACQUIRE SUBTROPICAL CHARACTERISTICS. THE SECOND FACTOR INVOLVES THE SURVIVABILITY OF THE LLCC. THE FRAGILE CIRCULATION COULD DISSIPATE BEFORE SUBTROPICAL TRANSITION IS COMPLETE. THE CURRENT FORECAST OF 36 HOURS IN DURATION MAY BE GENEROUS, AND IT IS QUITE POSSIBLE THAT THE CIRCULATION COULD DEVOLVE INTO AN OPEN WAVE BY TAU 24 OR SOONER. MODEL DISCUSSION: FOR THE FIRST TIME, NUMEROUS MODELS LOSE TRACK OF THIS WEAK, COMPACT CIRCULATION AND INADVERTENTLY LATCH ONTO THE STRONG VORTEX OF TS 20W AFTER TAU 12. THEREFORE, THE JTWC FORECAST TRACK IS PLACED ALONG A BLEND OF THE RELIABLE TRACKERS, HEDGED TOWARD THE PREVIOUS FORECAST TRACK. AVAILABLE INTENSITY GUIDANCE IS SIMILARLY SPARSE. THE JTWC INTENSITY FORECAST ALIGNS WITH THE ESTABLISHED FORECAST PHILOSOPHY, POSITIONED CLOSEST TO THE SHIPS STATISTICAL-DYNAMICAL MODEL AND THE COAMPS-TC SOLUTION INITIALIZED WITH NAVGEM BOUNDARY CONDITIONS. FORECAST CONFIDENCE: TRACK 00-72 HR: LOW INTENSITY 00-72 HR: MEDIUM// NNNN