California NWS Watches, Warnings, and Advisories and Landfalling Atmospheric Rivers during 2007–2016

California (CA) is frequently influenced by landfalling atmospheric rivers (ARs) that may produce high-impact weather such as flooding, heavy precipitation, and mountain snowfall. The goal of this research is to determine the fraction of high-impact weather days that are associated with landfalling ARs during a 10-year study from 2007–2016 in CA. The methodology employs an analysis of the spatial distribution of National Weather Service-issued watches, warnings or advisories (WWAs) over CA and their association with landfalling ARs. The “control” spatial analyses illustrate that liquid WWAs (e.g., floods and flash floods) are issued during both the cold season and warm season, whereas frozen WWAs (e.g., blizzards and winter storms) are primarily issued in the winter over the Sierra Nevada; flash flood WWAs are primarily issued in the warm season in southern CA.

WWAs in northern and southern California are issued on days with landfalling ARs approximately 60–80% and 40–60% of the time with a peak during the cool-season, respectively. WWAs in either region are much less likely to be issued on days with landfalling ARs in the warm-season, especially over southern CA. A composite analysis of the days with the highest number of WWAs issued over northern and southern CA illustrate an elongated plume of enhanced integrated water vapor (IWV) and IWV transport (IVT) consistent with a cool-season landfalling ARs along the California Coast. A few members of the southern CA composite, however, occurred during the warm-season, which motivates future work on the role of the North American monsoon and WWAs.