Large-Scale Influences on Atmospheric River–Induced Extreme Precipitation Events along the Coast of Washington State

Transient, narrow plumes of strong water vapor transport, referred to as Atmospheric Rivers (ARs), are responsible for much of the precipitation along the west coast of the United States. Along the coast of Oregon and Washington, the most intense cool season precipitation events are almost always induced by an AR and can result in detrimental impacts on society due to mudslides and flooding. It is therefore important to understand the large scale influences on extreme AR events so that they can be accurately predicted over a range of timescales, perhaps up to seasonal. Here, using data from the Modern Era Retrospective analysis for Research and Applications, Version 2, characteristics of ARs impinging on the coast of Washington State are analyzed, with a focus on isolating the particular AR ‘flavors’ that result in extreme precipitation events there. In addition to more intense water vapor transport, the ARs that produce extreme precipitation events show notable differences in synoptic scale forcing, particularly an anomalously deep low pressure system stationed to the west in the Gulf of Alaska, alongside a jet streak overhead. Attention will also be given to subseasonal and seasonal teleconnection patterns that are known to influence the weather in the Pacific Northwest of the United States. While little influence can be seen from the phase of the El Niño Southern Oscillation, Pacific Decadal Oscillation, and Pacific North American Pattern, the Madden Julian Oscillation (MJO) does appear to have an impact on the strength of precipitation associated with ARs on the Washington Coast. Lastly, interactions between the MJO and other teleconnection patterns will be explored to determine key features that should be investigated when making subseasonal predictions for AR activity and the associated precipitation in the Pacific Northwest.