WDPN33 PGTW 261500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING NR 002// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 17.2N 161.3E INITIAL INTENSITY: 25 KTS GEOGRAPHIC REFERENCE: 327 NM WEST-SOUTHWEST OF WAKE ISLAND MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 04 KTS SIGNIFICANT WAVE HEIGHT: 10 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A DISORGANIZED CLUSTER OF PERSISTENT CONVECTION OBSCURING A VERY COMPACT LOW-LEVEL CIRCULATION CENTER. THE CONVECTIVE ACTIVITY HAS GOOD POLEWARD VENTILATION AMIDST A LOW SHEAR ENVIRONMENT. A CRUCIAL ASCAT-C PASS AT 261101Z REVEALED WINDS OF 30 KTS ASSOCIATED WITH A MINIATURE LLCC. THE ENVIRONMENT IS FAVORABLE, CHARACTERIZED BY 10-15 KTS OF SOUTHERLY VERTICAL WIND SHEAR (VWS) AND VERY WARM SEA SURFACE TEMPERATURES (SST) OF 30-31 DEGREES CELSIUS. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED ASCAT IMAGE. THIS IMAGE IS ALSO THE BASIS FOR RAISING THE INITIAL INTENSITY TO 30 KTS WITH HIGH CONFIDENCE. THE 30 KT ASSESSMENT IS CONSISTENT WITH THE T2.0 DVORAK FIXES FROM KNES AND PHFO AND THE MAJORITY OF OBJECTIVE ESTIMATES FROM CIMSS. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: NORTHWESTERN FLANK OF A LOW- TO MID-LEVEL NEAR-EQUATORIAL RIDGE (NER) AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.5 - 25 KTS KNES: T2.0 - 30 KTS PHFO: T2.0 - 30 KTS CIMSS ADT: 29 KTS AT 261210Z CIMSS AIDT: 31 KTS AT 261210Z CIMSS D-MINT: 35 KTS AT 260749Z CIMSS D-PRINT: 28 KTS AT 261140Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 10-15 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE JTWC FORECAST TRACK HAS SHIFTED EASTWARD BY 40-50 NM FROM THAT OF THE PREVIOUS WARNING. FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 20W IS MEANDERING IN A WEAKENED STEERING ENVIRONMENT CAUSED BY THE CIRCULATION OF 91W LOCATION ROUGHLY 450 NM TO THE EAST-NORTHEAST. BOTH OF THESE CIRCULATIONS WILL BE PICKED UP BY AN EXTENSIVE LOW- TO MID-LEVEL NER, INDUCING A NORTHEASTWARD TRAJECTORY OVER THE NEXT 24 HOURS. AFTER THAT POINT, A SUBTROPICAL RIDGE CENTERED OVER THE DATELINE WILL DRIVE THE SYSTEM NORTHWARD. THERE REMAINS SUBSTANTIAL FORECAST UNCERTAINTY. TWO DISTINCT SCENARIOS ARE POSSIBLE. THE FIRST IS THAT TD 20W WILL BECOME ABSORBED BY THE LARGER CIRCULATION OF 91W, WHICH WOULD RESULT IN A SIGNIFICANTLY SHORTER FORECAST. THE SECOND IS THAT ENOUGH SEPARATION BETWEEN THE TWO CIRCULATIONS WILL EXIST THAT ALLOWS BOTH TO SURVIVE AS DISTINCT SYSTEMS. THE JTWC FORECAST CURRENTLY FOLLOWS THE LATTER SCENARIO. ASSUMING THAT TD 20W WILL SURVIVE THE INTERACTION WITH 91W, A SLOW TO GRADUAL DEVELOPMENT TREND IS LIKELY IN LARGE PART DUE TO LOW VWS AND WARM SST THROUGH TAU 48. THE SYSTEM WILL INTENSIFY MORE QUICKLY BETWEEN TAU 48 AND TAU 72 ONCE ITS INNER CORE BECOMES BETTER ESTABLISHED. INCREASING VWS AND SIGNIFICANTLY LOWER OCEAN HEAT CONTENT WILL LEAD TO GRADUAL WEAKENING BEYOND TAU 72. THROUGH THE FORECAST PERIOD, TD 20W WILL MAINTAIN A VERY COMPACT WIND FIELD. THE SIZE OF THE CIRCULATION PRESENTS AN ADDITIONAL CHALLENGE THAT IS DIFFICULT TO PREDICT IN ADVANCE. MESOSCALE PHENOMENA LIKE OUTFLOW BOUNDARIES, SUBSIDENCE, AND MARGINAL INCREASES IN VWS AS A RESULT OF THE NEARBY 91W CAN SUBSTANTIALLY INFLUENCE THE DEVELOPMENT TREND OF TD 20W. MODEL DISCUSSION: THE TRACKER GUIDANCE IS IN GOOD AGREEMENT FOR THE FIRST 24 HOURS, BECOMING POOR BEYOND TAU 48 DUE TO UNCERTAINTIES ASSOCIATED WITH THE INTERACTION OF 91W, WHICH MAY ALSO BE STRENGTHENING. IT IS DIFFICULT TO DISCERN WHICH MODELS PREDICT ABSORPTION BY 91W AND WHICH DO NOT, GIVEN THAT THE MAJORITY SHOWS SIMILAR TRACKS. NONETHELESS, THE JTWC FORECAST TRACK IS POSITIONED NEAR THE MULTI-MODEL CONSENSUS (CONW) WITH LOW CONFIDENCE. LIKEWISE, A LARGE SPREAD EXISTS AMONG THE INTENSITY AIDS, WITH THE STATISTICAL-DYNAMICAL SHIPS MODEL PEAKING THE SYSTEM AT 70 KTS, HAFS-A AT 60 KTS, COAMPS-TC AT 50 KTS, AND GFS AT 45 KTS. INTERESTINGLY, THE SHIPS RAPID INTENSIFICATION (RI) INDEX, FRIA, RIDE, AND THE CIMSS RI-AI SHOW ELEVATED POTENTIAL FOR RAPID DEVELOPMENT. GIVEN THE SPREAD AND MIXED SIGNALS, THE JTWC INTENSITY FORECAST IS SET ESSENTIALLY ON THE CONSENSUS WITH LOW CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: LOW TRACK 72-120 HR: LOW INTENSITY 00-72 HR: LOW INTENSITY 72-120 HR: LOW// NNNN