Synoptic Analysis of Cold Air Outbreaks over the California Central Valley

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How does extreme cold air reach the California Central Valley (CCV) and most of the U.S. west coast? This question is answered using composite patterns for the 10 coldest cold air outbreaks (CAOs) to reach the CCV during 1979–2013. While unusually cold air over California occurs in all events by design, how it arrives there is complicated and varies. The only other feature present in all events for several days prior to CAO onset is unusually strong surface high pressure in and south of the Gulf of Alaska. This high has low-level cold air on its west side and a deep layer of cold air moving southward on its east side. Cold air aloft flows parallel to the North American west coast and sinks as it approaches the CCV. Farther west, warm advection builds a ridge aloft. The large-scale meteorological pattern (LSMP) is equivalent barotropic. The LSMP’s ridge over Alaska, trough near California, and ridge over the southeastern United States appear in all cases by onset and resemble the Pacific–North American teleconnection pattern. Cross sections show cold air flowing from the continental interior consistent with a strong pressure gradient created by extreme cold in the continental interior. Where and when the interior cold and surface flow occurs varies between events. A geopotential height trough associated with that cold air aloft passes over the CCV before onset fostering sinking behind that is reinforced by the cold air advection below. Although sinking, as a locally defined anomaly, the cold intensifies as it migrates from the polar region to the climatologically warmer CCV.