Divergent Responses of Historic Rain-on-Snow Flood Extremes to a Warmer Climate

Global warming is expected to shift high-impact flood risks induced by rain-on-snow (ROS). However, decision-makers lack guidance on how impactful events resembling historic ROS extreme floods with destructive consequences would unfold in a warmer climate. Using a kilometer-scale storyline analysis, we examine warming-induced changes in four historically-significant, decision-relevant ROS extreme flood events over the contiguous United States. The runoff of different extreme events shows divergent responses to a warmer climate due to altered water budget. Notably, the basin-wide runoff of 2017-Jan California flood event (2017CA-Jan) increases by ~53%/K. Warming shifts flood generating regimes along the elevation profile. Driven by increasing rainfall and snowmelt, high elevations could experience significantly increased runoff and unprecedented floods in a warmer climate. For 2017CA-Jan, runoff increases at ~162%/K over the regions above 1500 m. Low elevations encounter a shift in the flood regime from ROS-driven to rainfall-dominated due to increasing rainfall and diminished snowmelt. These underscore needs for a warming-adaptive, regime-customized strategy for flood control.