The Role of Rain‐on‐Snow in Flooding Over the Conterminous United States.

Based on a process‐level modeling of the rain‐on‐snow (ROS) events in the period of 1950 to 2013 and in a warmer climate, we quantify the historical and future runoff contribution from ROS to extreme floods and the source of runoff and snowmelt in large ROS events within the conterminous United States (CONUS). We find that the regions impacted most heavily by ROS include the West Coast, the major mountain ranges of the western interior, the Upper Midwest, the Northeast, and the lower Appalachians. While 70% of extreme (upper 0.1%) runoff events in these regions have some contribution from ROS, the runoff generated during these ROS events accounts for less than 10% of the total extreme flood runoff; the much larger fraction of extreme runoff is from either intense rainfall or clear‐sky snowmelt. Rainfall is the dominant source of runoff in ROS events along the West Coast and over the west‐facing slopes of the Cascades and Sierra Nevada, while snowmelt dominates ROS runoff in the other regions in the CONUS. Net radiation dominates the snowmelt during ROS in the high mountains in the West, while net radiation and turbulent heat flux are equally dominant in the rest of CONUS. Historically, the role of ROS in streamflow extremes is most significant in midelevation areas, but this "significant influence zone" will shift to higher elevations in a warmer future. The future ROS frequency changes exert a first order control on the future change of the runoff contribution from ROS to extreme floods. Plain Language Summary: Rain‐on‐snow (ROS) occurs during periods when liquid precipitation falls on a pre‐existing snowpack. The intense rainfall and the rapid snowmelt during large ROS events have a long history of contributing to floods that lead to costly property damages and even the loss of life. For example, the 2017 Oroville Dam crisis that caused damages exceeding $1B was caused in part by ROS flooding. Through an analysis of long‐term ROS and flood records over the conterminous United States, we evaluate the contribution of ROS to past flooding, and how ROS's contribution to floods will change in a warmer future. Historically, ROS has contributed to many historical flood events across the conterminous United States, especially at low to intermediate elevations (between 1,000m and 1,500 m). However, its overall contribution to the total flood runoff mostly is modest. Radiative energy, primarily the longwave radiation emitted from the moist atmosphere, collectively explains the most snowmelt during ROS. In a warmer future, the contribution of ROS to floods runoff will generally increase at high elevations (above 2,000 m) in the Western United States and will mostly decrease in the rest of the conterminous United States. Key Points: Rain on snow affects many flood events across the conterminous United States, but its contribution to total flood runoff mostly is modestRain‐on‐snow frequency exerts a first‐order control on the role of rain on snow in hydrologic extremesNet radiation is overall the most dominant energy source for the snowmelt during rain on snow in most of the conterminous United States [ABSTRACT FROM AUTHOR]