WDPN32 PGTW 122100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NAMTHEUN) WARNING NR 012// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 21.0N 151.6E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 234 NM SOUTHWEST OF MINAMI TORI SHIMA MOVEMENT PAST 6 HOURS: NORTHWARD AT 11 KTS SIGNIFICANT WAVE HEIGHT: 17 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY INDICATES THAT THE PREVIOUSLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) HAS TUCKED UP UNDER THE WESTERN EDGE OF THE CIRRUS SHIELD ASSOCIATED WITH A FLARE-UP OF DEEP CONVECTION. A PARTIAL 121753Z SSMIS 91GHZ MICROWAVE IMAGE SHOWED THAT THE BULK OF THE CONVECTIVE MASS IS SHEARED MORE THAN 60NM TO THE NORTHEAST OF THE LLCC DUE TO PERSISTENT SOUTHWESTERLY SHEAR. AGENCY FIXES WERE WIDELY SCATTERED IN A 80NM CIRCLE AND THE INITIAL POSITION, PLACED NEAR THE CENTROID OF THE AGENCY FIX POSITIONS AND NEAR THE PGTW POSITION, WAS PLACED WITH ONLY MEDIUM CONFIDENCE. THE INITIAL INTENSITY OF 35 KNOTS IS ASSESSED WITH HIGH CONFIDENCE, BASED ON A CONGRUENCE AT T2.5 FROM ALL AGENCIES, AND THE ADT ESTIMATE OF 32 KNOTS. THE ENVIRONMENT IS MARGINAL AT BEST, WITH PERSISTENT SOUTHWESTERLY SHEAR OFFSETTING STRONG POLEWARD OUTFLOW AND WARM SSTS. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT BETWEEN A SUBTROPICAL RIDGE (STR) DISPLACED WELL NORTH OF THE CLIMATOLOGICAL POSITION, AND A STR TO THE SOUTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T2.5 - 35 KTS KNES: T2.5 - 35 KTS CIMSS ADT: 32 KTS AT 121740Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 15-20 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: HIGH INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TROPICAL STORM 23W HAS TURNED SHARPLY POLEWARD OVER THE PAST 12 HOURS, INDICATING THAT THE STR TO THE SOUTHEAST HAS STARTED TO ASSUME THE DOMINANT STEERING ROLE. GFS AND ECMWF GLOBAL FIELDS ARE HINTING THAT THE RIDGE TO THE NORTH IS BEGINNING TO ERODE WHILE EVACUATING THE AREA TO THE EAST. THE RECENT TURN NORTHWARD SEEMS TO SUPPORT THIS IS IN FACT OCCURRING AT A FASTER RATE THAN ANTICIPATED. THE TRACK IS THUS UPDATED WITH THIS FORECAST, NOW TURNING MORE SHARPLY TO THE NORTH THEN NORTHEASTWARD ALONG THE NORTHWEST PERIPHERY OF THE RIDGE TO THE SOUTHEAST, NOW THAT IT HAS ESTABLISHED AS THE PRIMARY STEERING MECHANISM. SHEAR IS FORECAST TO RELAX SLIGHTLY OVER THE NEXT 12 TO 24 HOURS AS THE TUTT CELL CURRENTLY TO THE WEST OF THE MARIANAS MOVES FURTHER WEST AND RELAXES THE UPPER-LEVEL SOUTHERLY FLOW OVER THE SYSTEM, ALLOWING FOR A SHORT PERIOD OF INTENSIFICATION TO A PEAK OF 40 KNOTS BY TAU 24. THE HONEYMOON WON'T LAST LONG HOWEVER, AND SHEAR WILL INCREASE ONCE AGAIN AFTER TAU 36, LEADING TO STEADY WEAKENING THROUGH THE END OF THE FORECAST. EXTRA-TROPICAL TRANSITION IS NOT EXPECTED AT THIS TIME AS THE SYSTEM IS FORECAST TO REMAIN WELL SOUTH OF THE BAROCLINIC ZONE AND THE SYSTEM IS EXPECTED TO DISSIPATE BELOW WARNING CRITERIA NO LATER THAN TAU 96. MODEL DISCUSSION: DUE TO THE COMPLEX AND EVOLVING STEERING PATTERN, TRACK GUIDANCE IS WIDELY SPREAD EVEN IN THE NEAR-TERM, WITH THREE GROUPINGS. THE FIRST GROUPING OF THE UKMET, GFS ENSEMBLE AND GALWEM TRACKERS ARE LAGGING BEHIND AND NOT REFLECTING THE RECENT MOTION TRENDS AND TAKE THE SYSTEM IN A WIDE POLEWARD TURN. THE NAVGEM, GFS, ECMWF ENSEMBLE MEANWHILE REFLECT A SHARPER TURN POLEWARD FROM TAU 00. THE REMAINDER OF THE GUIDANCE TAKES THE MIDDLE ROAD. THE JTWC FORECAST LIES ALONG THE EASTERN SIDE OF THE MIDDLE ENVELOPE, CLOSER TO THE INSIDE TRACK GROUPING, AND REMAINS ON THE RIGHT SIDE OF THE ENVELOPE THROUGH THE FORECAST PERIOD. INTENSITY GUIDANCE ALSO SHOWS A FAIR AMOUNT OF SPREAD, WITH THE HWRF AND COAMPS MODELS INSISTING ON MODERATE INTENSIFICATION, WHILE THE REMAINDER ARE FLAT OR SHOW WEAKENING FROM TAU 00. THE JTWC FORECAST IS ABOVE THE CONSENSUS MEAN THROUGH TAU 24, THEN SLIGHTLY BELOW FOR THE REMAINDER OF THE FORECAST PERIOD. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN