A climatological study of National Weather Service watches, warnings and advisories in association with atmospheric rivers in the western U.S. 2006-18

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Bartlett, Samuel M.
A recently published scale to characterize the intensity and duration of atmospheric rivers (AR) by Ralph et al. (2019) summarizes a range of typical impacts associated with these storms from weak and beneficial to strong and hazardous. This study investigates the spatial and temporal relationship between landfalling ARs along the U.S. West Coast and potentially hazardous weather as identified by National Weather Service watches, warnings, and advisories (WWAs) during 2006-18. The results of this study re-affirm that landfalling ARs occur in association with various types of high-impact weather including high wind, hydrologic extremes (e.g., heavy rainfall and flooding), and cold precipitation (e.g., snow) by presenting qualitative relationships between NWS products and ARs. Across the western U.S., 30-70% of days with any type of WWA were associated with landfalling ARs of any intensity. In the northern (Pacific Northwest) and central (Northern California and Great Basin) regions hydrologic-related and wind-related WWAs were more frequently (40-80%) associated with more intense and longer duration ARs classified as AR3, AR4, or AR5 events according to the Ralph et al. (2019) scale as compared to less intense and shorter duration AR1 or AR2 events. This relationship decreases for cold precipitation-related WWAs, where stronger ARs were more likely to result in rain than snow. While an approximate majority of days with WWAs occurred in association with landfalling ARs, not all landfalling ARs are associated with high-impact weather. For example, days with landfalling ARs occurred in association with WWA days of any type on 20-50% of cool-season days across high-elevation regions of the northern Cascades, Sierra Nevada, and southern California coastal ranges. This association increased to 40-60% for more intense and longer duration AR3, AR4, and AR5 events, especially for landfalling ARs in southern California and hydrologic-related WWAs over the southwest U.S. These results are summarized by an investigation of the WWA hazard footprint association with landfalling ARs that illustrates a quasi-exponential increase in the average cumulative area headlined by WWAs across the Western Region as the intensity and duration of a landfalling AR increases from an AR1 to an AR5 event, particularly for landfalling ARs in California and for hydrologic- and wind-related WWAs.