Wintertime extreme precipitation events and flooding along the Pacific Northwest 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.