Atmospheric Rivers and National Weather Service Watches, Warnings, and Advisories Issued Over California 2007–2016

Atmospheric rivers (ARs) are a focus of many global hydrometeorological studies and applications. The impacts of ARs along the United States’ West Coast include extreme orographic precipitation that often leads to flooding, flash flooding, debris flows, and other hydrological hazards that necessitate issuance of watches, warnings, and advisories (WWAs) by the National Weather Service (NWS). The objectives of this paper are to quantify and illustrate the relationship between landfalling ARs and high-impact weather events in California by comparing a catalog of landfalling ARs to a 10-year geospatial catalog of WWAs issued by the NWS. NWS WWAs are issued frequently over California in regions of topography (e.g., the northern Sierra Nevada and Transverse Ranges) in association with flooding and winter weather across northern California and flash flooding across southern California. A large majority of cool-season days with WWAs related to flooding (~50– 75%) and winter weather (~60–80%) occur on days with landfalling ARs. Alternatively, landfalling ARs on cool-season days also enhance the likelihood of high-impact weather over different regions of California with greatly increased likelihoods of WWAs relative to climatology as AR intensity increases. ABSTRACT (Manuscript received 26 July 2018; review completed 12 October 2018)