WDPN33 PGTW 270300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING NR 004// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 16.7N 161.4E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 335 NM WEST-SOUTHWEST OF WAKE ISLAND MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 06 KTS SIGNIFICANT WAVE HEIGHT: 12 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A COMPACT AND FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) WHICH HAS UNEXPECTEDLY MOVED SOUTHEASTWARD OVER THE PAST SIX HOURS. DEEP CONVECTION HAS WANED OVER THE PAST SIX HOURS, AND BEEN PERSISTENTLY DISPLACED TO THE NORTH DUE TO MODERATE SOUTHERLY MID-LEVEL SHEAR. HOWEVER, IN THE LAST HOUR, DEEP CONVECTION HAS ONCE AGAIN BEGUN TO FORM VERY NEAR THE LLCC, PARTICULARLY FOCUSED ON THE NORTHERN FLANK. CIMSS DEEP-LAYER SHEAR ANALYSIS SHOWS LIGHT SOUTHERLY SHEAR, BUT MORE MODERATE MID-LEVEL SHEAR, WHICH SUPPORTS THE SATELLITE DEPICTION. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN THE MSI. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, BASED ON A BLEND OF THE OBJECTIVE AND SUBJECTIVE INTENSITY ESTIMATES NOTED BELOW. ENVIRONMENTAL CONDITIONS ARE MARGINAL, CHARACTERIZED BY WEAK OUTFLOW ALOFT, LOW DEEP-LAYER AND MODERATE MID- LEVEL SHEAR AND WARM SSTS. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: WEAK STEERING PATTERN. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS RJTD: T1.5 - 25 KTS PHFO: T2.0 - 30 KTS CIMSS ADT: 35 KTS AT 262340Z CIMSS AIDT: 36 KTS AT 262340Z CIMSS D-MINT: 24 KTS AT 261958Z CIMSS D-PRINT: 26 KTS AT 262340Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 5-10 KTS SST: 29-30 CELSIUS OUTFLOW: WEAK POLEWARD OTHER FACTORS: BINARY INTERACTION WITH TD 21W. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 72 HOURS AND NOW CALLS FOR THE SYSTEM TO DISSIPATE BY MEANS OF ABSORPTION INTO TD 21W. FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 20W UNEXPECTEDLY TRACKED SOUTHEASTWARD OVER THE PAST SIX HOURS. WHILE THE SPECIFIC FORECAST DID NOT ANTICIPATE THIS TRACK VECTOR, THE 12Z AND 18Z ECMWF MODEL DID A GOOD JOB PICKING IT UP AND APPEARS TO HAVE INITIALIZED THE BEST OF ALL THE GLOBAL MODELS. GOING FORWARD, THE STEERING GRADIENT IS EXPECTED TO RAPIDLY SHIFT FROM THE WEST SIDE OF THE CIRCULATION TO THE SOUTHERN FLANK, ALONG THE NORTHERN EDGE OF A BAND OF STRONG SOUTHWESTERLY MONSOONAL WINDS WHICH EXTENDS FROM GUAM TO EAST OF TD 21W. TD 20W IS ALREADY STARTING TO TURN EASTWARD BASED ON THE LATEST MSI FRAMES, AND THIS TREND IS EXPECTED TO CONTINUE IN THE SHORT-TERM, THOUGH SOME ADDITIONAL ERRATIC MOTION IS LIKELY OVER THE NEXT FEW HOURS, ESPECIALLY IF THE RECENTLY DEVELOPED CONVECTION CAN STICK AND THE LLCC ATTEMPTS TO ALIGN. TD 20W IS FORECAST TO TURN GRADUALLY NORTHEAST, THEN NORTH AFTER TAU 24 AS IT RIDES UP THE WEST SIDE OF AN EXTENSION OF THE STR WHICH PUSHES SOUTHWARD ALONG THE DATELINE. AT THIS POINT, THE SYSTEM WILL BEGIN DIRECT BINARY INTERACTION WITH TD 21W, AND IS EXPECTED TO BE FULLY CAPTURED BY 21W BETWEEN TAU 36 AND TAU 48. ONCE CAPTURED, TD 20W WILL FOLLOW IN THE LEE OF 21W AND BEGIN TO EMBED WITHIN ITS WIND FIELD. REGARDING INTENSITY, 20W IS CURRENTLY 415NM SOUTHWEST OF TD 21W, FAR ENOUGH DISTANT THAT THE OUTFLOW ASSOCIATED WITH 21W IS NOT DIRECTLY IMPACTING 20W YET. MODEL FIELDS SUGGEST A REDUCTION IN SHEAR IN THE NEAR-TERM, WHICH SHOULD ALLOW FOR A PERIOD OF INTENSIFICATION AFTER TAU 12. TD 20W WILL HOWEVER STRUGGLE WITH LIMITED OUTFLOW ALOFT AND THE INFLUENCE OF DRY AIR AS WELL AS BURSTS OF OUTFLOW INDUCED SHEAR FROM 21W, WHICH WILL INHIBIT DEVELOPMENT. THE PEAK OF 45 KNOTS IS EXPECTED TO OCCUR AROUND TAU 48, BY WHICH POINT 20W WILL BEGIN TO BE ABSORBED INTO 21W. THE SYSTEM QUOTE DISSIPATES UNQUOTE BY TAU 72 AS IT IS FULLY INTEGRATED INTO THE WIND FIELD OF 21W THOUGH IT IS EXPECTED TO RETAIN AN ASYMMETRIC WIND FIELD OF AT LEAST 30 KNOTS AS IT DOES SO. MODEL DISCUSSION: TRACK GUIDANCE IS IN POOR AGREEMENT, THOUGH THE EXACT VALUES OF CROSS-TRACK SPREAD BECOME DIFFICULT TO ASCERTAIN AFTER TAU 24, AS THE MODELS ARE BEGINNING TO JUMP CIRCULATIONS BETWEEN 20W AND 21W AS THEY UNDERGO BINARY INTERACTION, CAPTURE AND MERGER. IN GENERAL HOWEVER, MODEL SPREAD BETWEEN THE GFS AND NAVGEM OPENS UP TO AT LEAST 500NM BY TAU 72, WITH EVEN HIGHER SPREAD BETWEEN THE NAVGEM AND THE EC-AIFS. THE BULK OF THE GUIDANCE HOWEVER SUPPORTS A GRADUAL TURN NORTHWARD IN THE WAKE OF TD 21W, INCLUDING THE ECMWF, WHICH NOTABLY SHIFTED TRACK POLEWARD THOUGH THIS MAY BE AN ARTIFACT OF IT JUMPING CIRCULATIONS. THE JTWC FORECAST IS PLACED NEAR THE MULTI-MODEL CONSENSUS MEAN, WITH VERY LOW CONFIDENCE. INTENSITY GUIDANCE IS SUFFERING FROM THE SAME ISSUES OF CIRCULATION JUMPING AS THE TRACK GUIDANCE, WHICH RESULTS IN THE MODELS DEPICTING A SHARP INTENSIFICATION TREND, WHICH IS EXTREMELY UNLIKELY. THE HAFS-A SEEMS TO BE THE ONLY MODEL WITH A GOOD HANDLE ON THE OVERALL SITUATION, DEPICTING SOME INTENSIFICATION FOLLOWED BY A GRADUAL WEAKENING BEFORE IT TOO SUCCUMBS TO THE JUMPING TREND. THE JTWC FORECAST IS PLACED NEAR THE HAFS-A SOLUTION THROUGH TAU 48 THEN DEVIATES SHARPLY LOWER THAN ALL THE GUIDANCE MEMBERS, WITH LOW CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: LOW INTENSITY 00-72 HR: LOW// NNNN