![]() ![]() We demonstrate this ambiguity in a case study of an impactful rain-on-snow event in California’s northern Sierra Nevada during 2017. ![]() However, current observational networks can mislead efforts to understand the key elements of such events (rainfall, snowmelt, and how water travels through snow). Sharpening our conceptual understanding of basin-scale ROS better equips water managers moving forward to appropriately classify threat levels, which are projected to increase throughout the mid-21st century.Įxtreme rain-on-snow can cause severe flooding. Given the breadth in plausible ROS flood mechanisms, this case study underscores a need for more detailed measurements of soil moisture along with in-storm changes to snowpack structure, extent, energy balance, and precipitation phase to address ROS knowledge gaps associated with current observational limits. Our analysis reveals a crucial link between frequent winter storms and a basin’s hydrologic response-emphasizing the role of soil moisture “memory” of within-season storms in priming impactful flood responses. A series of hydrologic model experiments and subdaily snow, soil, streamflow, and hydrometeorological measurements demonstrate that direct, “passive” routing of rainfall through snow, and increasingly efficient runoff driven by gradually wetter soils can alternatively explain the extreme runoff totals. ![]() However, we present evidence that snowmelt may have played a smaller role than previously documented (augmenting terrestrial water inputs by 21%). Prior studies have suggested that snowmelt during ROS dramatically amplified reservoir inflows. One such sequence of events preceded the evacuation of 188,000 residents below the already-damaged Oroville Dam spillway in February 2017 in California’s Sierra Nevada. This precipitation shift also invites more high-impact rain-on-snow (ROS) events, which have historically yielded many of the largest and most damaging floods in the western United States. Mountain snowpacks are transitioning to experience less snowfall and more rainfall as the climate warms, creating more persistent low- to no-snow conditions. ![]()
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