Atmospheric Rivers and Floods in California’s Changing Hydroclimate

September 15, 2025

The paper titled Atmospheric Rivers and Floods in California’s Changing Hydroclimate was co-authored by SIO/CW3E’s Sasha Gershunov, Alexander Weyant, Michael Dettinger, Lu Su, Dan Cayan, Rosa Luna Niño, Kristen Guirguis, Tom Corringham and Marty Ralph, as well as seven other scientists and water management practitioners, including colleagues at DWR. It was recently published in a special issue of San Francisco Estuary and Watershed Science. It presents the state of the science on atmospheric rivers, extreme precipitation and flood risk in the changing climate, as impacts California’s water management systems.

The article was commissioned by California’s Delta Stewardship Council that acts as the steward for sustainable and ecologically-responsible management of California’s water as part of the State of Bay-Delta Science (SBDS) synthesis report. This regular and ongoing effort is intended to inform the scientific and water management communities about the state of the science relevant to California’s water and its management¹. The 2025 SBDS report focused on extreme weather and climate events. Although meant as a review paper of current research on the co-evolution of atmospheric rivers (ARs) and flood risk in the changing climate of the State, this paper also presents original research results on the expected trends in extreme precipitation event probabilities and a look ahead into how extreme precipitation will be conveyed into hydrological impacts as the climate warms and snowpack erodes. The paper concludes with specific recommendations, and is meant to inform the evolution of California’s water policy over the next few decades.

Summarizing the state of the science on ARs and intense precipitation driving regional floods, the article explains why climate-model projections indicate increasing contributions to extreme precipitation from ARs, and an increasingly variable hydroclimate, with increased risk of floods. It also describes how observations do not yet show enhanced precipitation intensity trends. In agreement with climate-model projections, observations 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 applies a holistic and universal probability model for precipitation events (Weyant et al, 2025) to downscaled CMIP6 data and shows how events of the magnitude associated with the New Year 1997 floods – the last truly catastrophic floods in the Bay Delta – are expected to become twice as likely by the late 21st century (see Figure 1 below). Moreover, as extreme precipitation events are expected to become wetter, original hydrologic modeling suggests that extreme runoff events will be disproportionately enhanced, primarily as the result of a greater fraction of rain vs. snow.

¹ https://sbds.deltacouncil.ca.gov : The SBDS is an ongoing synthesis and communication effort intended to inform science and policy audiences about the “state of the science” for topics relevant to managing the Bay-Delta system. Editions of SBDS consist of collections of scientific articles published in the peer-reviewed journal San Francisco Estuary and Watershed Science (SFEWS). While the first two editions of SBDS (in 2008 and 2016) covered a broad range of topics, recent editions contain smaller suites of articles focused on key issues identified by the SBDS Editorial Board.
The 2025 edition features seven articles exploring extreme climate and weather events and their impacts on the Bay-Delta and its watershed. Individual chapters address heatwaves, droughts, atmospheric rivers, wildfires, and related governance and climate adaptation considerations. In the final chapter, the editorial board provides perspectives on advancing science and management to meet the needs of a changing Delta facing increasingly frequent climate and weather extremes.

Figure 1. (Fig. 4 from Gershunov et al. 2025) provides an overview of the NY 1997 precipitation event assessed in observations and in climate projections (see paper for details). This result applies the trivariate probability distribution (TED, Weyant et al. 2025) model for precipitation events. The three random variables defining physical events are duration, maximum intensity and overall magnitude. For precipitation events, this is a natural universal model where duration is a random variable and intensity is appropriately heavy tailed. TED allows us to estimate probabilities and return periods of very specific precipitation events and we’re already applying it to downscaled climate projections to estimate the changing probabilities of impactful historical events and out-of-sample events in an evolving climate. (A) Map of the Yuba–Feather watershed, subdivided at the hydrological unit code 8 (HUC86). (B) Progression of WY 1997 in the sub-watersheds; all values are relative to the climatological average precipitation accumulated from the beginning of the water year through the beginning of the NY 1997 event, 1 October through 29 December. (C) Observed daily precipitation time-series in the upper Yuba around NY 1997; the event-defining threshold of 19.7 mm/day is plotted as a horizontal line segment, and the NY 1997 event has grey shading behind it. (D) Box plots of estimated return periods of an event total precipitation that exceeded the total observed during the NY 1997 event (472 mm). In the construction of the box plot, there is one data point per climate model, with 14 GCMs used. See article for details.

Gershunov, A., Hatchett, B., Weyant, A., Dettinger, M., Su, L., Rhoades, A., Williams, P., Anderson, M., Rittelmeyer, P., Lettenmayer, D., Cayan, D., Niño, R. L., Guirguis, K., Corringham, T., Maendly, R., & Ralph, F. M. (2025). Atmospheric Rivers and Floods in California’s Changing Hydroclimate. San Francisco Estuary and Watershed Science, 23(3). http://dx.doi.org/10.15447/sfews.2025v23iss3art3

Weyant, A., Gershunov, A., Panorska, A. K., Kozubowski, T. J., & Kalansky, J. (2025). A holistic stochastic model for precipitation events. Scientific Reports, 15, 4595. https://doi.org/10.1038/s41598-024-77031-3