WDPN33 PGTW 270900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING NR 016// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 35.8N 142.3E INITIAL INTENSITY: 45 KTS GEOGRAPHIC REFERENCE: 130 NM EAST-NORTHEAST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: NORTHWARD AT 09 KTS SIGNIFICANT WAVE HEIGHT: 20 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 11W (NEPARTAK) REMAINS ASSESSED AS A SUBTROPICAL STORM. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS SHALLOW BANDING WRAPPING INTO A VERY WELL-DEFINED AND FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). ANIMATED WATER VAPOR IMAGERY, UPPER-AIR OBSERVATIONS AND MODEL ANALYSIS INDICATE THAT THE LLCC ASSOCIATED WITH 11W IS CURRENTLY UNDER A COLD UPPER-LEVEL LOW WITH DRY AIR ADVECTING INTO THE CIRCULATION, PARTICULARLY ACROSS THE SOUTHERN HEMISPHERE. THE CIRA THERMAL CROSS-SECTION ANALYSIS VALID AT 270000Z CONFIRMS THE WARM OVER COLD ANOMALY STRUCTURE ASSOCIATED WITH SUBTROPICAL SYSTEMS. SCATTERED AND DISORGANIZED CONVECTION IS OCCURRING IN THE NORTHERN SECTORS OF THE CIRCULATION UNDER THE COLD POOL ALOFT, WHICH IS SLIGHTLY OFFSET FROM THE UPPER-LEVEL LOW CENTER. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC, SUPPORTED BY ANIMATED RADAR DATA AND RJTD RADAR FIX POSITIONS. THE INITIAL INTENSITY IS ASSESSED WITH MODERATE CONFIDENCE, HEDGED HIGHER THAN THE SUBTROPICAL DVORAK INTENSITY ESTIMATES OF T2.5 FROM PGTW, IN LIGHT OF THE HIGHER SATCON VALUES AND PERSISTENCE WITH EARLIER SCATTEROMETER DATA. ADDITIONALLY, SURFACE PRESSURES FROM CHOSHI, JAPAN BOTTOMED OUT AT 984 MB, WHICH SUPPORTS A CENTRAL PRESSURE IN THE 988 MB RANGE, EQUIVALENT TO APPROXIMATELY 45 KTS. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: MID-LEVEL, NORTH TO SOUTH ORIENTED SUBTROPICAL RIDGE (STR) CENTERED TO THE EAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T2.0 - 30 KTS CIMSS SATCON: 51 KTS AT 262139Z CIMSS ADT: 43 KTS AT 270600Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE VWS: 0-5 KTS SST: 26-27 CELSIUS OUTFLOW: WEAK WESTWARD OTHER FACTORS: WHILE VWS IS LOW AND SSTS ARE HIGH, THE OVERALL ENVIRONMENT IS UNFAVORABLE FOR TROPICAL CYCLONE DEVELOPMENT DUE TO THE PRESENCE OF THE UPPER-LEVEL LOW OVER THE LLCC. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: AFTER A SHORT PERIOD OF UNEXPECTED WEST-SOUTHWEST MOVEMENT EARLIER IN THE DAY, 11W IS NOW FIRMLY ENTRENCHED ON A NORTHWESTWARD TRACK. THE SYSTEM IS EXPECTED TO MAKE LANDFALL IN THE VICINITY OF SENDAI, JAPAN WITHIN THE NEXT 12 TO 15 HOURS, THEN TRANSIT QUICKLY ACROSS THE NORTHERN PORTION OF HONSHU BEFORE REEMERGING INTO THE SEA OF JAPAN NEAR TAU 24. THE SYSTEM WILL THEREAFTER TURN ONTO A MORE WESTWARD TRACK WHILE SLOWING RAPIDLY AS IT MOVES INTO A WEAK STEERING ENVIRONMENT. THE SYSTEM IS FORECAST TO WEAKEN SLIGHTLY PRIOR TO LANDFALL, BUT IS STILL EXPECTED TO BE A SUBTROPICAL STORM AT LANDFALL. CONTINUED WEAKENING IS FORECAST AS IT CROSSES THE JAPANESE ALPS, REEMERGING OVER WATER AS A 30 KT SUBTROPICAL DEPRESSION. AS THE SYSTEM CONTINUES TO MOVE WEST THROUGH TAU 72, THE UPPER-LEVEL LOW WHICH HAS BEEN SHIELDING THE SYSTEM FROM HIGH VWS WILL BEGIN TO SHIFT BACK EASTWARD, ALLOWING THE CIRCULATION ASSOCIATED WITH 11W TO MOVE UNDER AN AREA OF VERY HIGH VWS ASSOCIATED WITH A 200 MB JET MAX, LEADING TO RAPID WEAKENING AND DISSIPATION NO LATER THAN TAU 72, AND POTENTIALLY AS EARLY AS TAU 48. MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 36 WITH A SPREAD OF 95 NM BETWEEN THE EXTREME OUTLIERS HWRF AND JGSM. HOWEVER THE JTWC FORECAST TRACK LIES CLOSE TO BUT SLIGHTLY SLOWER THAN THE MULTI-MODEL CONSENSUS MEAN AND AMONGST THE TIGHT GROUPING OF THE MAJORITY OF THE CONSENSUS MEMBERS. AFTER TAU 36, THE MODELS SHOW SIGNIFICANT DIVERGENCE IN SOLUTIONS, WITH THE JGSM AND ECMWF ENSEMBLES SHOWING A TURN TO THE NORTH-NORTHEAST, WHILE THE HWRF AND GALWEM INDICATE THE WEAKENING CIRCULATION TURNING SHARPLY SOUTHWARD AFTER TAU 36. THE REMAINDER OF THE CONSENSUS MEMBERS SUPPORT THE JTWC FORECAST TRACK TO THE WEST THROUGH TAU 72, THOUGH WITH ONLY MEDIUM CONFIDENCE. THE JTWC INTENSITY FORECAST LIES CLOSE TO, BUT SLIGHTLY LOWER, THAN THE INTENSITY CONSENSUS MEAN THROUGH THE DURATION OF THE FORECAST PERIOD, WITH MEDIUM CONFIDENCE DUE TO THE UNCERTAINTIES ASSOCIATED WITH THE INFLUENCE OF TERRAIN INTERACTION AND THE ULTIMATE STRENGTH OF THE LLCC THAT EMERGES INTO THE SEA OF JAPAN. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM// NNNN