Wintertime Extreme Precipitation Events along the Pacific Northwest Coast: Climatology and Synoptic Evolution

Extreme precipitation events impact the Pacific Northwest during winter months, causing flooding, landslides, extensive property damage, and loss of life. Outstanding questions about such events include whether there are a range of associated synoptic evolutions, whether such evolutions vary along the coast, and the associated rainfall duration and variability. To answer these questions, this study uses 60 years of National Climatic Data Center (NCDC) daily precipitation observations to identify the top 50 events in two-day precipitation at six coastal stations from northern California to northwest Washington. NCEP–NCAR reanalysis data were used to construct synoptic composite evolutions of these events for each coastal location. Most regional flooding events are associated with precipitation periods of 24 h or less, and two-day precipitation totals identify nearly all major events. Precipitation areas of major events are generally narrow, roughly 200 km in width, and most are associated with atmospheric rivers. Composite evolutions indicate negative anomalies in sea level pressure and upper-level height in the central Pacific, high pressure anomalies over the southwest United States, large positive 850-hPa temperature anomalies along the coast and offshore, and enhanced precipitable water and integrated water vapor fluxes over southwest to northeast swaths. A small subset of extreme precipitation events over the southern portion of the domain is associated with a very different synoptic evolution: a sharp trough in northwesterly flow and post-cold-frontal convection. High precipitable water values are more frequent during the summer, but are not associated with heavy precipitation due to upper-level ridging over the eastern Pacific and weak onshore flow that limit upward vertical velocities.