WDPN33 PGTW 262100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING NR 003// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 17.2N 161.0E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 343 NM WEST-SOUTHWEST OF WAKE ISLAND MOVEMENT PAST 6 HOURS: EASTWARD AT 01 KTS SIGNIFICANT WAVE HEIGHT: 12 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN EXTREMELY COMPACT AND DISORGANIZED SYSTEM, WITH A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) DISPLACED TO THE SOUTH OF FLARING DEEP CONVECTION AND ASSOCIATED MID-LEVEL MESOVORTEX. A 261524Z AMSR2 36GHZ IMAGE REVEALED THE COMPACT LLCC EXPOSED ON THE SOUTHERN EDGE OF A WEDGE OF LINEAR CONVECTION EXTENDING NORTHWARD. BASED ON COMPARISON OF THE AMSR2 IMAGERY AND THE EARLIER ASCAT DATA, THE SYSTEM HAS REMAINED QUASI-STATIONARY SINCE 1200Z. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AMSR2 DATA AND ANIMATED MSI. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A BLEND OF THE AVAILABLE SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES NOTED BELOW. THE ENVIRONMENT IS MARGINALLY FAVORABLE, CHARACTERIZED BY LOW DEEP- LAYER WIND SHEAR, MODERATE POLEWARD OUTFLOW ALOFT, AND WARM SSTS WITH HIGH OHC. THE SYSTEM IS APPROXIMATELY 415NM SOUTHWEST OF INVEST 91W AND UNDERGOING THE INITIAL PHASE OF BINARY INTERACTION. THE TWO SYSTEMS ARE INTER-CONNECTED WITHIN A 1009MB CLOSED OUTER ISOBAR. THE TWO ARE ALSO INTERTWINED ALOFT, WITH 20W POSITIONED IN THE SOUTHWESTERN END OF AN ELONGATED UPPER-LEVEL ANTICYCLONE CENTERED ROUGHLY IN THE VICINITY OF INVEST 91W. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT; WEAK BAND OF LOW-LEVEL SOUTHWESTERLIES ALONG THE NORTHWESTERN PERIPHERY OF A NEAR EQUATORIAL RIDGE (NER) AND BINARY INTERACTION WITH INVEST 91W TO THE NORTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.0 - 25 KTS KNES: T2.0 - 30 KTS PHFO: T2.5 - 35 KTS CIMSS SATCON: 34 KTS AT 261720Z CIMSS ADT: 34 KTS AT 261740Z CIMSS AIDT: 32 KTS AT 261740Z CIMSS D-MINT: 22 KTS AT 261751Z CIMSS D-PRINT: 22 KTS AT 261800Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 5-10 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE POLEWARD OTHER FACTORS: EXTREMELY COMPACT NATURE OF THE SYSTEM AND THE INFLUENCE OF THE BINARY INTERACTION WITH 91W. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM 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) 20W HAS MOVED LITTLE IN THE PAST SIX HOURS, CURRENTLY TRAPPED IN A WEAK AND COMPETING STEERING ENVIRONMENT. OVER THE NEXT 12 TO 24 HOURS, TD 20W WILL TRACK SLOWLY EASTWARD ALONG THE NORTH SIDE OF THE WEAK NER TO THE SOUTHEAST. AFTER TAU 24, THE NER IS EXPECTED TO RECEDE EQUATORWARD, WHILE A NOSE OF THE STR TO THE NORTHEAST PUSHES SOUTHWARD TO THE EAST OF BOTH 20W AND 91W. THE COMBINATION OF A STRENGTHENING GRADIENT ON THE EASTERN SIDE OF TD 20W AND THE INFLUENCE OF THE BINARY INTERACTION WITH 91W, WILL PUSH TD 20W NORTHWARD THROUGH THE REMAINDER OF THE FORECAST PERIOD. REGARDING INTENSITY, THE SYSTEM WILL REMAIN STAGNANT OR INTENSIFY ONLY SLIGHTLY OVER THE COURSE OF THE NEXT 24 HOURS DUE TO THE CONTINUED INFLUENCE OF MODERATE MID-LEVEL SOUTHERLY SHEAR AND A RELATIVELY WEAK OUTFLOW ENVIRONMENT ALOFT. BY TAU 36, AS 91W MOVES RAPIDLY NORTHWARD INTO A BAND OF UNFAVORABLE CONDITIONS ALOFT, 20W WILL HAVE AN OPPORTUNITY TO INTENSIFY MORE RAPIDLY AS IT REMAINS IN A REGION OF LIGHT SHEAR AND MODERATE POLEWARD OUTFLOW. 20W STEADILY INTENSIFIES TO A PEAK OF 60 KNOTS BY TAU 60 BEFORE IT TOO SUCCUMBS TO INCREASING SHEAR AND DRY AIR ENTRAINMENT, AND BEGINS TO WEAKEN THROUGH THE REMAINDER OF THE FORECAST. THAT IS FORECAST DOOR NUMBER ONE. BEHIND DOOR NUMBER TWO LIES AN ALTERNATE SCENARIO, ONE THAT SEES TD 20W DISSIPATED WITHIN THE NEXT 24 HOURS. AS DEPICTED BY THE ECMWF, INVEST 91W MOVES NORTHWARD AT A MUCH SLOWER PACE, WHILE TD 20W MOVES EAST-NORTHEASTWARD AT A FASTER PACE. THIS RESULTS IN TD 20W BECOMING ABSORBED WITHIN THE MUCH LARGER WIND FIELD ASSOCIATED WITH 91W AS IT TRACKS SLOWLY NORTHWARD. AT THIS POINT, THE VAST MAJORITY OF MODEL GUIDANCE SUPPORTS THE CURRENT FORECAST WITH ONLY THE ECMWF AND GOOGLE DEEPMIND SUPPORTING THE ALTERNATE SCENARIO. HOWEVER, IT CANNOT BE DISMISSED ENTIRELY AND WILL BE CLOSELY MONITORED. MODEL DISCUSSION: TRACK GUIDANCE IS MIXED. AS DISCUSSED ABOVE, THE MAJORITY OF THE MODEL GUIDANCE DEPICTS A NORTHEASTWARD TRACK THROUGH TAU 24, THEN A TURN NORTHWARD THROUGH THE REMAINDER OF THE FORECAST. THE ECMWF AND FNV3 ARE THE TWO MODELS SUPPORTING AN EASTWARD TRAJECTORY FOR JUST 24 HOURS, BEFORE DISSIPATING THE SYSTEM. THE GFS OFFERS A MIDDLE-ROAD SOLUTION, EJECTING NORTHEASTWARD AFTER TAU 48 AND APPROACHING THE DATELINE BY TAU 120. THE PRIMARY GUIDANCE ENVELOPE (DISREGARDING GFS, ECMWF AND FNV3) DISPLAYS A 140NM CROSS-TRACK SPREAD, WHICH INCREASES TO 230NM BY TAU 120. ALONG-TRACK SPREAD INCREASES SHARPLY AFTER TAU 48, TO 350NM BY THE END OF THE FORECAST PERIOD. ENSEMBLE MODELS SHOW A VAST AMOUNT OF MEMBER DISPERSION, BASICALLY COVERING THE ENTIRETY OF THE NORTHWEST PACIFIC BASIN, MAKING THIS FORECAST EXTREMELY UNCERTAIN. THE JTWC FORECAST IS PLACED NEAR THE AI CONSENSUS MEAN, WITH VERY LOW CONFIDENCE. INTENSITY GUIDANCE IS GENERALLY IN AGREEMENT ON THE OVERALL INTENSITY TREND, BUT DIFFERS SIGNIFICANTLY IN THE PEAK INTENSITY. THE HAFS-A IS THE MOST AGGRESSIVE MODEL, PEAKING THE SYSTEM AT 85 KNOTS AT TAU 72, WHILE THE COTC MODEL IS THE LEAST AGGRESSIVE, REACHING A MAXIMUM OF 45 KNOTS. THE JTWC FORECAST IS PLACED NEAR THE CONSENSUS MEAN, BUT AGAIN WITH VERY LOW CONFIDENCE IN LIGHT OF THE ALTERNATE SCENARIO OUTLINED ABOVE. FORECAST CONFIDENCE: TRACK 00-72 HR: LOW TRACK 72-120 HR: LOW INTENSITY 00-72 HR: LOW INTENSITY 72-120 HR: LOW// NNNN