| Summary: | In December 2020, the Western North Pacific was active with extratropical low-pressure weather events. During the final week of 2020, there was a record-breaking 1092 millibar high-pressure system over Mongolia, and 24 hours later, a 921 millibar extratropical low-pressure system over Western Alaska. These two extremes were associated with pressure anomalies for the basin and a jet superposition of the polar and subtropical jet streams. Superposition events are rare but occur in this region during winter. The Climate Forecast System Reanalysis (CFSR) data was used to examine jet superposition using 200 and 300 millibar winds and potential vorticity channels or waveguides in relation to the development of extratropical cyclones. Results indicated that PV channeling from Siberia in a northern airstream combined with jet superposition between the subtropical and polar jets resulted in rapid intensification. Using PV channels and their merging as an indicator could be a valuable forecast tool for extratropical cyclone intensification. Analyzing events such as these will allow forecasters to better prepare Naval assets in the Western Pacific for extreme events. Lieutenant, United States Navy Approved for public release. Distribution is unlimited.
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