A Climatological Study of National Weather Service Watches, Warnings, and Advisories and Landfalling Atmospheric Rivers in the Western United States 2006-18

Atmospheric rivers (ARs) are synoptic-scale phenomena associated with long, narrow corridors of enhanced low-level water vapor transport. Landfalling ARs may produce numerous beneficial (e.g., drought amelioration and watershed recharge) and hazardous (e.g., flash flooding and heavy snow) impacts that may require the National Weather Service (NWS) to issue watches, warnings, and advisories (WWAs). Prior research on WWAs and ARs in California found that 50%-70% of days with flood-related and 60%-80% of days with winter weather-related WWAs occurred on days with landfalling ARs in California. The present study further investigates this relationship for landfalling ARs and WWAs during the cool seasons of 2006-18 across the entire western United States and considers additional dimensions of AR intensity and duration. Across the western United States, regional maxima of 70%-90% of days with WWAs issued for any hazard type were associated with landfalling ARs. In the Pacific Northwest and central regions, flood-related and wind-related WWAs were also more frequently associated with more intense and longer-duration ARs. While a large majority of days with WWAs were associated with landfalling ARs, not all landfalling ARs were necessarily associated with WWAs (i.e., not all ARs are hazardous). For example, regional maxima of only 50%-70% of AR days were associated with WWAs issued for any hazard type. However, as landfalling AR intensity and duration increased, the association with a WWA and the "hazard footprint" of WWAs increased quasi-exponentially across the western United States. SIGNIFICANCE STATEMENT Atmospheric rivers are frequently associated with hazardous weather across the western United States necessitating watches, warnings, and advisories by the National Weather Service. This study quantifies the relationship between atmospheric rivers and watches, warnings, and advisories for different types of hazards (e.g., flooding, high winds, winter weather) and for different atmospheric river characteristics (e.g., intensity and duration). A large majority of watches, warnings, and advisories are associated with atmospheric rivers; however, not all atmospheric rivers necessarily require a watch, warnings, or advisory. The more intense and longer-duration atmospheric rivers are also more likely to require a watch, warning, or advisory.