Wintertime extreme precipitation events and flooding along the Pacific Northwest coast

Extreme precipitation events over the west coast of North America have a large societal impact, causing landslides and flooding that result in millions of dollars of property damage and potential loss of life. These extreme events are related primarily to “atmospheric rivers” of high moisture content from the tropics and subtropics that reach the western U.S. Although much work has been done to understand the nature of extreme precipitation events along the west coast of the United States, many important questions remain. How long do extreme precipitation events last? Does the synoptic evolution for each event look the same, or are there other ways besides ARs to get extreme precipitation? Does the synoptic evolution vary as you move south along the coast? Are there ARs that occur off shore that are not associated with extreme precipitation along the coast?

These “rivers” of moisture in combination with the right synoptic conditions can produce heavy precipitation over 12-72 hours, with most lasting 24-48 hours. Therefore, a combination of hourly and daily precipitation observation data over the last 60 years for six coastal stations from northern California to Washington are used to find the highest 48-hr precipitation totals during the rainy season from October to March. The results are the top 50 storms at each station, or 300 days in the last 60 years. Although each station exhibits a large amount of variability in the shape and amplitude of the associated moisture field in each storm, nearly all heavy precipitation events are related to ARs and share similar characteristics in other synoptic fields. These characteristics remain similar and progress south as you move south along the coast. However, a small subset of storms which occurred at stations to the south have different, non-AR, synoptic signatures. It is believed that a combination of post-frontal convection and orographic effects are responsible for an extreme event with relatively low moisture content off shore. Additionally, ARs occur off the West Coast without a significant amount of precipitation upon landfall. In fact, the highest moisture values off the coast occur in October but the majority of storms occur in NDJ, evidence that synoptic support, not just high moisture content, is a necessary ingredient for extreme precipitation.