WDPN33 PGTW 280900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 21.9N 132.2E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 366 NM SOUTHEAST OF KADENA AB MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 14 KTS SIGNIFICANT WAVE HEIGHT: 12 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL DEPRESSION 20W (TWENTY) HAS QUICKLY CONSOLIDATED OVER THE PREVIOUS 12 HOURS WHILE STEADILY INTENSIFYING. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS DEEP FLARING CONVECTION FIRING PREDOMINANTLY IN THE NORTHERN SECTORS OF THE CIRCULATION, THOUGH IT REMAINS DISORGANIZED IS NOT EVIDENCED TO BE WRAPPING INTO THE LOW LEVEL CIRCULATION CENTER (LLCC) AS OF YET. DUE TO A LACK OF RECENT MICROWAVE IMAGERY, THE INITIAL POSITION IS ASSESSED WITH ONLY MEDIUM CONFIDENCE USING SPIRAL BAND ANALYSIS OF THE OUTER LOW-LEVEL CLOUD LINES SEEN TO BE WRAPPING INTO THE PERIPHERY OF THE SYSTEM. THE INITIAL INTENSITY IS ASSESSED AT 30 KNOTS WITH HIGH CONFIDENCE, HEDGED SLIGHTLY ABOVE THE AGENCY FIX INTENSITIES BASED ON AN EARLIER 280124Z ASCAT-B PARTIAL PASS WHICH SHOWED A LARGE REGION OF 30 KNOT WINDS IN THE NORTHERN PERIPHERY OF THE SYSTEM. ADDITIONALLY A 280300Z SHIP OBSERVATION, JUST 20NM FROM THE 0600Z POSITION, REGISTERED A 1003MB PRESSURE AND WINDS OF 32 KNOTS. THE SYSTEM LIES IN A FAVORABLE ENVIRONMENT WITH LOW (0-5 KNOTS) VWS, MODERATE RADIAL OUTFLOW WITH A POINT SOURCE OVER TOP OF THE SYSTEM AND WARM SSTS. THE ONLY INHIBITING FACTOR AT PRESENT IS THE DISORGANIZED CORE STRUCTURE OF THE SYSTEM. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED TO THE EAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.5 - 25 KTS KNES: T1.5 - 25 KTS FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 0-5 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 20W WILL CONTINUE TO TRACK NORTH-NORTHWESTWARD THROUGH THE NEXT 12 HOURS, REACH THE AXIS OF THE STEERING RIDGE SHORTLY THEREAFTER AND THEN TURN SHARPLY NORTHEAST AND THE STR FLATTENS AND ELONGATES TO A MORE EAST-WEST ORIENTATION. ONCE THE SYSTEM CAN CONSOLIDATE A SOLID CORE, A PERIOD OF INTENSIFICATION IS EXPECTED AS THE FAVORABLE CONDITIONS OF WARM SSTS, LOW SHEAR AND MODERATE RADIAL OUTFLOW ARE FORECAST TO CONTINUE THROUGH THE NEXT 36 HOURS AND THE SYSTEM SHOULD REACH PEAK INTENSITY OF 45 KNOTS BY TAU 36. BY TAU 48, A 500MB MINOR SHORTWAVE TROUGH IS FORECAST TO MOVE ACROSS THE JAPANESE ISLANDS AND LEAD TO CYCLOGENESIS OF A WEAK LOW PRESSURE AREA EAST OF TOKYO, ALONG THE TAIL END OF A COLD FRONTAL SYSTEM LEFT IN THE WAKE OF WHAT IS NOW TY 19W. THE UPPER TROUGH AND BAROCLINIC LOW WILL BE FAIRLY WEAK AND QUICKLY SLIDE TO THE EAST-NORTHEAST ON A RELATIVELY FLAT TRAJECTORY, LEAVING THE FRONTAL SYSTEM STALLED TO NORTH OF TD 20W. THE ONSET OF INCREASING SHEAR AROUND TAU 48 WILL MARK THE BEGINNING OF AN INITIALLY SLOW WEAKENING TREND. BY TAU 72 HOWEVER THE UPPER-LEVEL ANTICYCLONE CURRENTLY PROVIDING GOOD RADIAL OUTFLOW WILL SLIDE TO THE SOUTHWEST OF THE SYSTEM, AND BEGIN TO IMPART STRONGLY CONVERGENT FLOW ALOFT OVER THE TOP OF THE SYSTEM, LEADING TO A MORE PRONOUNCED WEAKENING PHASE. AS A NOW SIGNIFICANTLY WEAKER AND SHALLOWER SYSTEM, TD 20W WILL BUTT HEADS WITH MODERATELY STRONG NORTHERLY FLOW BEHIND THE COLD FRONT WILL EFFECTIVELY BLOCK FURTHER POLEWARD MOVEMENT. AS THE BAROCLINIC HIGH SLIDES EASTWARD THE LOW LEVEL WIND FLOW BECOMES EASTERLY, AND WILL PUSH TD 20W OFF THE WEST-NORTHWEST BY THE END OF THE FORECAST PERIOD. MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN FAIRLY GOOD AGREEMENT THOUGH TAU 48 WITH A 110NM SPREAD BETWEEN THE NAVGEM ON THE WEST AND GFS ON THE EAST. AFTER THIS POINT HOWEVER, THINGS GET ROUGH. AT THE TAU 72 POINT, THE MODELS BEGIN TO SEPARATE SHARPLY. AT TAU 72, THE ALONG-TRACK SPREAD BETWEEN THE NAVGEM AND JGSM TO THE WEST AND THE EGRR AND GALWEM TO THE EAST EXCEEDS 350NM. BY TAU 96 THE BULK OF THE GUIDANCE, WITH THE EXCEPTION OF GALWEM AND EGRR STARTS TO SHOW THE TURN TO THE WEST, CONTINUING THROUGH TAU 120. HOWEVER, THE MODELS DISAGREE ON THE SHARPNESS AND STARTING POINT OF THE TURN, LEADING TO A VERY LARGE SPREAD IN MODEL SOLUTIONS BAY TAU 120. THE JGSM AND HWRF LIE ON THE FAR WEST OF THE ENVELOPE, WHILE THE GFS ENSEMBLE MARKS THE EASTERN SIDE OF THE MODELS THAT SUPPORT THE TURN, WITH OVER 450NM OF SEPARATION BETWEEN THESE TWO OUTLIERS. THE REMAINDER OF THE GFS, JGSM, NAVGEM, COAMPS-TC, ECMWF ENSEMBLE AND THE CONSENSUS MEAN LIE EQUALLY SPREAD WITHIN THIS 450NM ENVELOPE. THE JTWC FORECAST TRACK LIES TO THE INSIDE AND SLIGHTLY FASTER THAN THE CONSENSUS MEAN TO TAU 72, THEN MOST CLOSELY TRACKS THE GFS ENSEMBLE MEAN TO THE END OF THE FORECAST. CONFIDENCE IS MEDIUM UP TO TAU 72 AND LOW THEREAFTER DUE TO THE UNCERTAINTIES ASSOCIATED WITH THE TIMING AND STRENGTH OF THE COLD FRONT AND TRANSITORY HIGH PRESSURE AREA AND THE ULTIMATE IMPACT THESE WILL HAVE ON THE SYSTEM. INTENSITY GUIDANCE IS IN GOOD AGREEMENT THAT THE SYSTEM WILL INTENSIFY THROUGH TAU 36 TO 48, THEN WEAKEN SLOWLY THEREAFTER. CTCX IS THE MOST AGGRESSIVE MODEL, PEAKING THE SYSTEM AT 52 KNOTS AT TAU 36, WHILE THE REMAINDER OF THE CONSENSUS MEMBERS LIE IN A TIGHT ENVELOPE WITH A PEAK BETWEEN 40-45 KNOTS. THE JTWC FORECAST LIES ALONG THE MEAN, WITH A SLIGHTLY HIGHER PEAK, THEN FOLLOWS THE MEAN DURING THE WEAKENING PHASE. CONFIDENCE IS LOW AS IF THE SYSTEM CAN QUICKLY CONSOLIDATE, A FASTER PACE OF INTENSIFICATION AND HIGHER PEAK IS CERTAINLY A POSSIBILITY. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: LOW INTENSITY 72-120 HR: LOW// NNNN