The hydrological significance of atmospheric rivers (ARs) remains less explored with regard to mountain snowpack. In this work, the contribution of ARs to the Sierra Nevada snowpack is quantified based on assimilated snow data. AR events contributed a large fraction of the seasonal total snowpack. On average 6–7 events per year provided 40% of the total snow water equivalent (SWE) over the 7 years studied (Fig.). A strong connection between AR-related snowpack changes and surface air temperatures is uncovered with in situ and satellite data, which helps better understand AR characteristics and impact mechanisms. By showing the sensitivity of snow-rain partitioning to surface temperatures during AR storms, the study highlights the potential impacts of projected climate change on regional hydrology and water resources. This work has been conducted under partnership with the CalWater experiment. Primary support is provided by ARRA funds, JPL, and NASA applications and terrestrial hydrology programs.