WDPN33 PGTW 240900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING NR 004// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 26.2N 150.0E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 755 NM SOUTHEAST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 10 KTS SIGNIFICANT WAVE HEIGHT: 15 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN ELONGATED LOW-LEVEL CENTER (LLC) EXPOSED WEST OF A RAGGED BLOB OF DEEP CONVECTION. 11W (NEPARTAK) IS A SUBTROPICAL CYCLONE, CONSISTING OF BOTH NON-TROPICAL AND TROPICAL FEATURES. MOIST SOUTHWESTERLY MONSOON FLOW IS WRAPPING AROUND THE EASTERN SIDE OF THE CYCLONE AND CAUSING AN OUTBREAK OF DEEP CONVECTION IN THE EASTERN SEMICIRCLE, BUT THE WESTERN SIDE OF THE CIRCULATION IS DRY THANKS TO A STRONG UPPER-LEVEL TROUGH TO THE NORTHWEST OF THE LLC, LOCATED SOUTHEAST OF TOKYO, JAPAN. NEPARTAK IS BEGINNING TO INTERACT BAROCLINICALLY WITH THIS TROUGH, WHICH IS INDUCING A BROAD, ASYMMETRIC WIND FIELD. A 240004Z ASCAT-B PASS SHOWED AN EXPANSIVE REGION OF GALES IN THE EASTERN SEMICIRCLE, WITH MAXIMUM WINDS LOCATED FAR FROM THE LLC (APPROXIMATELY 135 NM). THIS STRUCTURE IS CONSISTENT WITH THAT OF A SUBTROPICAL CYCLONE. THE INITIAL INTENSITY IS SET AT 35 KT BASED ON THE AFOREMENTIONED ASCAT PASS. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: UPPER-LEVEL LOW TO THE NORTHWEST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T1.5 - 25 KTS CIMSS SATCON: 28 KTS AT 240346Z CIMSS ADT: 29 KTS AT 240540Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 25-30 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE EASTWARD ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE FORECAST NO LONGER SHOWS DISSIPATION AT 96 HOURS, AND MAINTAINS 11W AS A SIGNIFICANT TROPICAL CYCLONE THROUGH 120 HOURS. FORECAST DISCUSSION: 11W (NEPARTAK) IS A SUBTROPICAL CYCLONE, CHARACTERIZED BY A BROAD, ASYMMETRIC WIND FIELD WRAPPING MONSOONAL MOISTURE AROUND THE EASTERN SIDE. THE CYCLONE IS BEGINNING THE PROCESS OF BAROCLINIC INTERACTION WITH THE STRONG UPPER-LEVEL LOW TO THE NORTHWEST, AND THIS INTERACTION WILL DOMINATE THE EVOLUTION OF NEPARTAK DURING THE FORECAST PERIOD. WITH THE MID-LATITUDE JET DISPLACED WELL NORTH OF JAPAN, THE AFOREMENTIONED UPPER-LEVEL LOW IS EXPECTED TO MOVE LITTLE OVER THE NEXT 48 HOURS, ALLOWING NEPARTAK TO PINWHEEL NORTHWARD AND THEN NORTHWESTWARD AROUND ITS PERIPHERY, EVENTUALLY REACHING JAPAN. THIS EVOLUTION WILL SHARE SIMILARITIES WITH BAROCLINIC OCCLUSION OF MID-LATITUDE CYCLONES, AND IS LIKELY TO CAUSE SOME INTENSIFICATION OF NEPARTAK DESPITE THE ELEVATED LEVELS OF VERTICAL SHEAR THAT ARE PRESENT. NUMERICAL MODEL GUIDANCE IS IN GENERAL AGREEMENT ON THIS POINT, AND THE JTWC INTENSITY FORECAST CALLS FOR MAXIMUM WINDS TO INCREASE TO 45 KT BY 36-48 HOURS. THEREAFTER, THE OCCLUSION PROCESS WILL COMPLETE, CONSISTING OF NEPARTAK BECOMING VERTICALLY ALIGNED BENEATH THE UPPER-LEVEL LOW. THIS EVOLUTION IS LIKELY TO CAUSE A CONTRACTION OF THE CYCLONE'S WIND FIELD AS DRY AIR WRAPS AROUND THE CIRCULATION, AND ALSO SOME WEAKENING AS NEPARTAK LOSES THE BAROCLINIC SUPPORT THAT THE UPPER TROUGH WAS PROVIDING PRIOR TO VERTICAL ALIGNMENT. NEPARTAK MAY ALSO PASS OVER COOLER SEA SURFACE TEMPERATURES (SST) JUST PRIOR TO LANDFALL IN NORTHERN HONSHU. THUS, SLIGHT WEAKENING IS FORECAST PRIOR TO LANDFALL BETWEEN 72 AND 96 HOURS. THE TRACK FORECAST BECOMES OF LOWER CONFIDENCE AFTER 72 HOURS, AS STEERING CURRENTS WILL WEAKEN ONCE NEPARTAK IS STACKED BENEATH THE UPPER-LEVEL LOW, MAROONED WELL TO THE SOUTH OF THE MID-LATITUDE JET. NUMERICAL MODEL GUIDANCE SUGGESTS THAT A SLOW DRIFT ACROSS HONSHU INTO THE SEA OF JAPAN IS CURRENTLY MOST LIKELY, AND THE JTWC FORECAST FOLLOWS SUIT. THE LONG-TERM INTENSITY FORECAST IS ALSO OF LOWER CONFIDENCE, AS LAND INTERACTION WITH JAPAN AND DRY AIR WRAPPED IN BY THE UPPER-LEVEL LOW COULD CAUSE DISSIPATION OF THE CYCLONE. HOWEVER, MOST MODEL GUIDANCE SHOWS A CIRCULATION SURVIVING THE CROSSING OF HONSHU AND EMERGING IN THE SEA OF JAPAN. IF THIS OCCURS, ANOMALOUSLY WARM SST OF 27-28C IN THE SEA OF JAPAN COULD ALLOW THE CYCLONE TO SLOWLY UNDERGO TROPICAL TRANSITION AND POTENTIALLY RESTRENGTHEN. THE JTWC FORECAST SHOWS NEPARTAK REGAINING TROPICAL STORM INTENSITY BY 120 HOURS, BUT REMAINS CONSERVATIVE, AS THIS PORTION OF THE FORECAST IS CURRENTLY OF LOW CONFIDENCE. MODEL DISCUSSION: THE JTWC TRACK FORECAST CLOSELY FOLLOWS THE MULTI-MODEL CONSENSUS, WHICH HAS SHIFTED SLIGHTLY TO THE RIGHT COMPARED TO THE PREVIOUS FORECAST. THE JTWC INTENSITY FORECAST IS ABOVE THE MULTI-MODEL CONSENSUS DURING THE FIRST 48 HOURS DUE TO THE ANTICIPATION THAT STRENGTHENING WILL OCCUR BY NON-TROPICAL PROCESSES THAT ARE NOT ACCOUNTED FOR IN SOME OF THE INTENSITY GUIDANCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN