Atmospheric rivers (ARs) are long (>2000 km) and narrow (500–1000 km) corridors of enhanced vertically integrated water vapor and enhanced integrated water vapor transport (IVT) that are responsible for a majority of global poleward moisture transport and can result in extreme orographic precipitation. Observational evidence suggests that ARs within different synoptic‐scale flow regimes may contain different water vapor source regions, orientations, and intensities and may result in different precipitation distributions. This study uses k‐means clustering to objectively identify different orientations and intensities of ARs that make landfall over the California Russian River watershed. The ARs with different orientations and intensities occur within different synoptic‐scale flow patterns in association with variability in IVT direction and quasi‐geostrophic forcing for ascent and lead to different precipitation distributions over the Russian River watershed. These differences suggest that both mesoscale upslope moisture flux and synoptic‐scale forcing for ascent are important factors in modulating precipitation distributions during landfalling ARs.

Southern California water resources are heavily dependent on a small number of extreme precipitation events each winter season, which dictate the region's highly variable interannual accumulations. In the Santa Ana River Watershed, on average, three extreme events per year contribute half of annual precipitation, yet there are relatively few studies of the synoptic to mesoscale processes that drive precipitation during these events. This study uses an ingredient‐based approach in identifying the contributions of orographic forcing, dynamical forcing, and convective instability to extreme precipitation in the watershed in 107 storms that produced roughly 50% of all precipitation from 1981 to 2017. The influence of dynamical forcing and convective instability on event precipitation distributions is investigated relative to the dominant influence of orographic forcing that is typically found in landfalling atmospheric rivers. Case studies of two high‐impact events from the 2017 winter season demonstrate differences in the roles of synoptic ascent and mesoscale convective features in modifying precipitation location, rate, and accumulation over the watershed. The 17 and 18 February 2017 case study included a narrow cold‐frontal rainband that produced high‐intensity short‐duration precipitation over low elevations of the watershed. In the 107 extreme event records, similar modification of the precipitation distribution toward non‐orographic rainfall was related to significant changes in the synoptic‐scale circulation that favored enhanced dynamics and upstream ascent associated with frontogenesis. Variability in precipitation mechanisms is of primary interest to weather forecasters and water managers as it modifies event impacts and predictability.

Capsule Atmospheric River Reconnaissance is a multi-year research and operations partnership to evaluate the potential of targeted airborne observations over the Northeast Pacific to improve forecasts of atmospheric river impacts on the U.S. West Coast at lead times of less than five days.