Dynamics of Extreme Precipitation Events in Northern California during Winter 2016–2017

This study examines planetary- and synoptic-scale dynamical processes that resulted in high-impact extreme precipitation events and culminated in record seasonal precipitation totals in northern California during the winter of 2016–2017. In particular, the two largest events (~4–10 January 2017 and ~3–11 February 2017), which collectively accounted for ~38% of the total seasonal precipitation volume over northern California, are investigated. These two events were found to have occurred within similar large-scale flow patterns over the North Pacific, featuring a major high-latitude blocking ridge over the central North Pacific and an active subtropical storm track over the eastern North Pacific. Results indicate that the blocking patterns were established and maintained in connection with successive Rossby wave packets propagating southeastward across Eurasia into the western North Pacific. These wave packets resulted in successive strong cyclogenesis, cyclonic wave breaking, and ridge amplification along the polar jet stream over the North Pacific and, additionally, induced downstream wave dispersion into the eastern North Pacific along the subtropical jet stream that “undercut” the blocking ridge. Interactions between undercutting subtropical disturbances and equatorward-propagating polar troughs linked to anticyclonic wave breaking on the eastern flank of the blocking ridge resulted in strong cyclogenesis over the eastern North Pacific and the concomitant formation of persistent atmospheric rivers that supported prolonged heavy precipitation over northern California. A brief climatological analysis is conducted to place the precipitation events under investigation into context and to investigate the degree to which the large-scale flow pattern associated with the events represents a principal dynamical pathway for the occurrence of extreme precipitation in northern California.