Linkages between extreme precipitation in northern California and atmospheric blocking over the North Pacific

Linkages between extreme precipitation in northern California and large-scale atmospheric blocking upstream over the North Pacific are investigated through statistical and composite analyses. The investigation employs 1979–2017 climatologies of extreme precipitation events, defined as extreme 7-day precipitation accumulations in northern California, and blocking ridges centered over the central North Pacific. A statistical analysis reveals that 1) an appreciable fraction (~33%) of the extreme precipitation events co-occur with blocking, and 2) the likelihood of extreme precipitation events occurring when blocking occurs is significantly enhanced compared to when blocking does not occur. Composite analyses and composite-based simulations elucidate multiscale processes whereby North Pacific blocking may support extreme precipitation events in northern California. It is found that anomalous water vapor flux into northern California, supporting the extreme precipitation, is established and maintained for >7 days as synoptic-scale disturbances propagate across the eastern North Pacific into the U.S. West Coast in rapid succession on the equatorward flank of a persistent blocking ridge. As these disturbances approach the coast, they interact with equatorward-propagating troughs established over northwestern North America and the Gulf of Alaska in conjunction with anticyclonic Rossby wave breaking on the downstream flank of the blocking ridge. Heavy precipitation is produced over northern California in conjunction with lifting linked to baroclinic forcing accompanying the landfalling disturbances and to local orographic forcing along elevated terrain. Brief case studies demonstrate that, owing to their prolonged nature, blocking-related precipitation events may result in extraordinary precipitation totals and severe flooding. Overall, the findings in this study demonstrate that blocking over the North Pacific represents a key large-scale dynamical pathway for the occurrence of high-impact extreme precipitation events in northern California.