WDPN33 PGTW 270900 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: 20.3N 126.3E INITIAL INTENSITY: 25 KTS GEOGRAPHIC REFERENCE: 381 NM SOUTH-SOUTHWEST OF KADENA AB MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 08 KTS SIGNIFICANT WAVE HEIGHT: 12 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL DEPRESSION (TD) 20W (TWENTY) HAS RAPIDLY ORGANIZED AND STRENGTHENED OVER THE PAST FEW HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC), ASSUMED TO BE IN THE CENTER OF ROTATING BANDS OF CONVECTION ORGANIZING INTO VORTICAL HOT TOWERS (VHTS) OVER LAST COUPLE OF HOURS. AN EARLIER SCATTEROMETER PASS INDICATED THE LLCC WAS HIDDEN UNDER THE NASCENT CONVECTION AT THAT TIME AND EXTRAPOLATION OF THE DATA POINT PROVIDED ADDITIONAL CLARITY TO THE INITIAL POSITION ESTIMATE. THE SYSTEM IS CLEARLY ORGANIZING AND BECOMING SYMMETRIZED, BUT THE 270416Z TROPICS-5 91GHZ PASS DEPICTING A NASCENT MICROWAVE EYE FEATURE MIGHT BE A BIT TOO MUCH. REGARDLESS, THE SYSTEM HAS THE LOOK. THE INITIAL POSITION WAS ASSESSED WITH MEDIUM CONFIDENCE BASED ON AN EXTRAPOLATION OF THE EARLIER SCATTEROMETER PASS AND ANALYSIS OF THE VISIBLE LOW-LEVEL CLOUD LINES. THE INITIAL INTENSITY WAS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE PGTW AND RJTD T1.5 DVORAK CURRENT INTENSITY ESTIMATES AND THE EARLIER SCATTEROMETER PASS WHICH SHOWED 25 KNOT WINDS OUTSIDE OF CONVECTION IN PATCHES ACROSS THE SOUTHERN AND WESTERN PORTIONS OF THE CIRCULATION. ANALYSIS REVEALS A FAVORABLE ENVIRONMENT, WITH VERY WARM SSTS, HIGH OHC, A SMALL POINT SOURCE ALOFT OVER THE SYSTEM, WITH EQUATORWARD OUTFLOW BEING ENHANCED BY AN UPPER-LEVEL TUTT-CELL TO THE EAST. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: EASTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED JUST SOUTH OF HONG KONG. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.5 - 25 KTS FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 5-10 KTS SST: 30-31 CELSIUS OUTFLOW: MODERATE POLEWARD AND EQUATORWARD 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: FOR THE NEXT 24 HOURS, THE PRIMARY STEERING MECHANISM FOR TD 20W WILL REMAIN THE STR TO THE WEST, CENTERED SOUTH OF HONG KONG. THE RIDGE HOWEVER IS NOT EXPECTED TO REMAIN STATIONARY BUT RATHER WILL QUICKLY EXIT STAGE RIGHT AND MOVE INTO NORTHERN VIETNAM, ALLOWING TD 20W TO TRACK SOUTHWESTWARD THROUGH TAU 24. AFTER TAU 24, THE SYSTEM MOVES INTO A WEAK STEERING PATTERN, CAUGHT BETWEEN A LARGE STR COMPLEX OVER SOUTHERN CHINA, AN EXTENSION OF THE STR EXTENDING ALONG A LINE FROM MINDANAO THROUGH GUAM AND TROUGHING TO NORTH OVER KOREA. SO FOR THE 24 HOURS FROM TAU 24 TO TAU 48, THE SYSTEM WILL VERY SLOWLY DRIFT WESTWARD AS THE VARIOUS RIDGES JOSTLE FOR DOMINANCE. BY TAU 48, THE STR EXTENSION TO THE SOUTHEAST IS FORECAST TO BREAK OFF INTO A DISCRETE ANTICYCLONE, AND START MOVING NORTHWARD WHILE CONTINUING TO BUILD. AFTER TAU 48, THIS NEW RIDGE CENTER WILL BECOME THE DOMINANT STEERING INFLUENCE, PUSHING TD 20W ONTO A GENERALLY NORTHWARD TRACK THROUGH THE REMAINDER OF THE FORECAST PERIOD. LATE IN THE FORECAST PERIOD, THE EXACT POSITION OF THE RIDGE CENTER RELATIVE TO TD 20W WILL HAVE A MAJOR IMPACT ON THE TRACK FORECAST AFTER TAU 96. THE FURTHER EAST THE CENTER, THE FURTHER EAST THE TRACK AND VICE VERSA. THE CURRENT FORECAST KEEPS THE CENTER EAST OF TAIWAN BUT SKIRTS THE EASTERN COAST AT A RELATIVELY CLOSE RANGE. IN TERMS OF INTENSITY, THE SYSTEM IS GOING TO ENJOY FAVORABLE CONDITIONS FOR THE BULK OF THE FORECAST PERIOD. IN THE NEAR-TERM, INTENSIFICATION WILL BE RELATIVELY SLOW AS THE SYSTEM CONTINUES TO CONSOLIDATE AND THE WIND FIELD SYMMETRIZES. THE PACE OF INTENSIFICATION IS EXPECTED TO PICK UP IN EARNEST AFTER TAU 36, AS BY THIS POINT THE CIRCULATION IS FULLY FORMED, IT IS MOVING SLOWLY OVER THE HIGHEST OHC ALONG THE TRACK AND THE ANTICYCLONE ALOFT BECOMES INTENSE, WITH THE SYSTEM EXPECTED TO REACH TYPHOON STRENGTH BY TAU 48, IF NOT SOONER. BUT THIS IS JUST SETTING THE STAGE FOR THE REAL FIREWORKS, WHICH COMMENCE AT TAU 48. RAPID INTENSIFICATION (IR) IS FORECAST BETWEEN TAU 48 AND TAU 72, FOLLOWED BY NEAR-RI UP TO TAU 96, WHEN THE SYSTEM IS EXPECTED TO PEAK AT OR NEAR 115 KNOTS. INTERACTION WITH TAIWAN, AND INCREASED SHEAR WILL RESULT IN MODEST WEAKENING BY TAU 120. ONE FACTOR THAT MAY INFLUENCE THE PEAK INTENSITY IS MODERATE TO HIGH MID-LEVEL SHEAR, WHICH THE GFS IN PARTICULAR IS PREDICTING TO IMPACT THE SYSTEM AROUND TAU 72. THE DETERMINISTIC GFS INTENSITY GUIDANCE PICKS UP ON THIS AND PEAKS THE SYSTEM AT ABOUT 95 KNOTS. MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN FAIRLY GOOD AGREEMENT ON THE OVERALL TRACK SCENARIO. ALL OF THE GUIDANCE SHOWS A SOUTHWEST TRACK, THEN SLOW WESTWARD MOVEMENT, FOLLOWED BY AN EJECTION TO THE NORTH BY TAU 48. THERE IS HOWEVER A MODEST AMOUNT OF CROSS-TRACK SPREAD BY TAU 72, WITH THE GALWEM SPLITTING OFF FROM THE PACK AND DRIVING THE SYSTEM OVER THE BABUYAN ISLANDS, WHILE THE GFS, GEFS AND ECMWF TURN THE SYSTEM MORE SHARPLY NORTHWARD, LEADING TO A 195NM SPREAD AT TAU 72. CROSS-TRACK SPREAD CONTINUES TO INCREASE SUCH THAT BY TAU 120, IT REACHES 330NM BETWEEN GALWEM NEAR KAOHSIUNG, TAIWAN, AND THE GEFS WEST OF KADENA. THE JTWC FORECAST LIES JUST INSIDE THE CONSENSUS MEAN TO TAU 72, THEN ALIGNS WELL WITH THE INTERPOLATED GFS AND ECWMF TO TAU 120. INTENSITY GUIDANCE IS IN STRONG AGREEMENT ON THE INTENSIFICATION TREND TO TAU 72, BEFORE THE MODELS SPLIT WITH THE AHNI PEAKING AT 95 KNOTS, WHILE THE HAFS-A PEAKS AT 125 KNOTS. THE JTWC FORECAST GENERALLY FOLLOWS THE CONSENSUS MEAN BUT PEAKS ABOUT 10 KNOTS HIGHER THAN THE MEAN. THE GEFS AND ECENS ENSEMBLE HAVE COME INTO BETTER AGREEMENT NOW, WITH BOTH MEANS KEEPING THE SYSTEM EAST OF TAIWAN, THOUGH THERE REMAINS A RELATIVELY LARGE SPREAD COVERING ROUGHLY THE TAIWAN STRAIT TO ISHIKAGIJIMA. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN