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Projected increases and shifts in rain-on-snow flood risk over western North America


Destructive and costly flooding can occur when warm storm systems deposit substantial rain on extensive snowcover1,2,3,4,5,6, as observed in February 2017 with the Oroville Dam crisis in California7. However, decision-makers lack guidance on how such rain-on-snow (ROS) flood risk may respond to climate change. Here, daily ROS events with flood-generating potential8 are simulated over western North America for a historical (2000–2013) and future (forced under Representative Concentration Pathway 8.59) period with the Weather Research and Forecasting model; 4 km resolution allows the basin-scale ROS flood risk to be assessed. In the warmer climate, we show that ROS becomes less frequent at lower elevations due to snowpack declines, particularly in warmer areas (for example, the Pacific maritime region). By contrast, at higher elevations where seasonal snowcover persists, ROS becomes more frequent due to a shift from snowfall to rain. Accordingly, the water available for runoff10 increases for 55% of western North American river basins, with corresponding increases in flood risk of 20–200%, the greatest changes of which are projected for the Sierra Nevada, the Colorado River headwaters and the Canadian Rocky Mountains. Thus, flood control and water resource planning must consider ROS to fully quantify changes in flood risk with anthropogenic warming.

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Fig. 1: In a warmer climate, ROS will be less frequent at lower elevations, particularly in warmer regions, and more frequent at middle elevations and in historically colder regions.
Fig. 2: Warming reduces ROS frequency at lower elevations due to snowpack loss and increases ROS frequency at higher elevations due to a shift from snowfall to rain.
Fig. 3: For many mountainous regions, future rain-on-snow events will be more intense, largely due to increases in rainfall rather than snowmelt increases.
Fig. 4: Increases in the ROS flood potential are projected, explained by a spatial expansion of ROS to include higher elevations and slight increases in rainfall intensity.


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The National Center for Atmospheric Research (NCAR) is sponsored by the National Science Foundation (NSF). K.N.M. was supported under an NCAR Advanced Study Program (ASP) Postdoctoral Fellowship. F.L. was supported by a Postdoc Applying Climate Expertise (PACE) fellowship co-sponsored by the National Oceanic and Atmospheric Administration and the Bureau of Reclamation, administered by Cooperative Programs for the Advancement of Earth System Science (CPAESS). The authors acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the NSF. The authors also thank N. Mizukami, A. Newman and E. Gutmann for discussions.

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All authors designed the study. K.N.M. conducted the analysis. C.L. ran the WRF simulations. K.I. managed the WRF output. M.B. customized the WRF output that facilitated ROS analysis. K.N.M., F.L., A.F.P. and M.P.C. contributed to the interpretations of the results. K.N.M., F.L., A.F.P., M.P.C. and R.R. wrote the paper.

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Correspondence to Keith N. Musselman.

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Musselman, K.N., Lehner, F., Ikeda, K. et al. Projected increases and shifts in rain-on-snow flood risk over western North America. Nature Clim Change 8, 808–812 (2018).

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