WDPN33 PGTW 261500 AMD MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING NR 013A AMENDED// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 35.2N 144.6E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 240 NM EAST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 19 KTS SIGNIFICANT WAVE HEIGHT: 15 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 11W (NEPARTAK) IS A SUBTROPICAL STORM UNDERGOING RAPID STRUCTURAL EVOLUTION DURING THE PAST SEVERAL HOURS. ANIMATED SHORTWAVE INFRARED AND WATER VAPOR SATELLITE IMAGERY SHOWS THAT NEPARTAK IS NOW CENTERED ALMOST SQUARELY BENEATH THE UPPER-LEVEL LOW (ULL) THAT IT HAS BEEN INTERACTING WITH FOR THE PAST FEW DAYS. DRY AIR ASSOCIATED WITH THIS ULL IS ADVECTING AROUND THE EAST AND NORTH SIDE OF NEPARTAK, CUTTING THE CIRCULATION OFF FROM THE CONVEYOR BELT OF MONSOONAL MOISTURE THAT IS NOW DISTANTLY REMOVED TO THE EAST. SOME OF THIS MOISTURE HAS BEEN WRAPPED AROUND THE WEST AND SOUTH SIDE OF NEPARTAK'S CENTER, WHERE A CLUMP OF DEEP CONVECTION EXISTS. THE LOW-LEVEL CENTER (LLC) ITSELF REMAINS MOSTLY EXPOSED TO THE NORTHEAST OF THIS CONVECTION, AND IS ELONGATED FROM SOUTHWEST TO NORTHEAST. SCATTEROMETER DATA FROM A 261036Z ASCAT-B PASS INDICATED THAT THE EXPANSIVE AREA OF STRONG WINDS PREVIOUSLY OBSERVED IN THE NORTHEAST QUADRANT IS WEAKENING, AND THE OVERALL WIND FIELD IS BECOMING MORE COMPACT, AS HAS BEEN EXPECTED. THE STRONGEST WINDS OF AROUND 40 KT ARE NOW LOCATED IN THE WESTERN SEMICIRCLE ACCORDING TO A 261219Z ASCAT-B PASS. CHOSHI, JAPAN OBSERVED A TROPICAL STORM FORCE SUSTAINED WIND OF 36 KT WITH A SEA-LEVEL PRESSURE OF 999 MB AT 261300Z. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: LOW TO MID-LEVEL RIDGE TO THE NORTH AND EAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS CIMSS SATCON: 39 KTS AT 260859Z CIMSS ADT: 26 KTS AT 260830Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 20-25 KTS SST: 27-28 CELSIUS OUTFLOW: WEAK WESTWARD ANALYSIS CONFIDENCE: INITIAL POSITION: LOW INITIAL INTENSITY: HIGH INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: 11W (NEPARTAK) IS A SUBTROPICAL CYCLONE NEARING COMPLETION OF AN OCCLUSION PROCESS THAT SHARES SIMILARITIES WITH SOME MID-LATITUDE CYCLONE LIFECYLES. AS THE LOW-LEVEL CENTER (LLC) HAS BECOME SECLUDED BENEATH THE UPPER-LEVEL LOW (ULL), AWAY FROM THE MONSOONAL MOISTURE BELT, THE SUPERPOSITION OF THE TWO VORTICES IS CAUSING A TEMPORARY SPIN-UP OF THE WIND FIELD WITHIN 200 NM OF NEPARTAK'S CENTER. THE CYCLONE SHOULD CURRENTLY BE AT OR NEAR PEAK INTENSITY WITH ITS WINDS OF 40 KT. NOW THAT THE LLC AND ULL ARE STACKED, BAROCLINIC FORCING HAS DISAPPEARED, SO SLIGHT WEAKENING OF NEPARTAK'S MAXIMUM WINDS TO 35 KT IS FORECAST IN 24 HOURS PRIOR TO LANDFALL IN JAPAN. GIVEN THE ELONGATED NATURE OF THE LLC, THE SHORT-TERM TRACK IS LIKELY TO CONSIST OF WOBBLES DURING THE NEXT 12 HOURS, FOLLOWED BY A MOVE NORTHWESTWARD TOWARD THE COASTLINE OF NORTHERN HONSHU, STEERED BY A DEEP-LAYER RIDGE TO THE EAST. LANDFALL IS FORECAST NEAR ISHINOMAKI, JAPAN IN JUST OVER 24 HOURS. NEPARTAK IS EXPECTED TO CROSS OVER INTO THE SEA OF JAPAN BY 36 HOURS, WEAKENED TO 30 KT BY LAND INTERACTION. FURTHER WEAKENING AND EVENTUAL DISSIPATION IS FORECAST BY 96 HOURS AS THE CYCLONE DRIFTS SLOWLY NORTHWARD INTO COOLER WATERS WITH SEA SURFACE TEMPERATURES OF LESS THAN 26 DEGREES CELSIUS. THE JTWC FORECAST FOR BOTH TRACK AND INTENSITY IS SIMILAR TO THE PREVIOUS FORECAST, EXCEPT A LITTLE FARTHER SOUTH DURING THE FIRST 12 HOURS TO ACCOUNT FOR RECENT CONSOLIDATION OF THE CIRCULATION TOWARD THE SOUTHWEST. MODEL DISCUSSION: THE JTWC TRACK FORECAST IS SLIGHTLY SOUTH OF THE MULTI-MODEL CONSENSUS THROUGH 12 HOURS, THEN LEANS A LITTLE TOWARD THE GFS AND GFS ENSEMBLE DUE TO THEIR QUALITY REPRESENTATION OF NEPARTAK'S CURRENT VORTEX STRUCTURE. THE JTWC INTENSITY FORECAST IS WITHIN 5 KT OF THE MULTI-MODEL CONSENSUS. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: HIGH INTENSITY 72-120 HR: MEDIUM// NNNN