Atmospheric Rivers and Floods in California’s Changing Hydroclimate

Flooding in the Bay–Delta is most commonly due to runoff from atmospheric river (AR) storms, often enhanced by low-elevation snowmelt. In this paper, we review the current science of ARs and their projected enhancement in a warming climate. We also address the changing state of the Sierra Nevada snowpack. Climate-model projections indicate increasing contributions to extreme precipitation from ARs, and more variable hydroclimate, with increased floods as well as droughts. Observations, meanwhile, do not yet show enhanced precipitation intensity trends. In agreement with climate-model projections, observations do show that, as the climate continues to warm, California’s greatest natural freshwater reservoir—its snowpack—continues to erode. This is despite record snowpacks (e.g., 2023) still being possible, and potentially exacerbating flood effects from ARs in a highly variable hydroclimate. Original analysis of extreme historical and projected precipitation events shows the occurrence of the magnitude at the level associated with the New Year 1997 floods are expected to become twice as likely by the late 21st century. Moreover, as extreme precipitation events are expected to become wetter, hydrologic modeling suggests that extreme runoff events will be disproportionately enhanced, primarily as the result of a greater fraction of rain vs. snow. We also discuss the mitigating influence of water management on extreme flows, and mention new research results, challenges, and opportunities associated with sub-seasonal and seasonal precipitation predictability. We suggest that—along with infrastructural modernization, as well as maintenance and improvement of observational networks—current and future challenges for water management can be mitigated by better and longer lead-time weather and climate-forecast information.