In the Sierra Nevada, midwinter "rain-on-snow" events occur when rain falls onto existing snowpack, and have resulted in some of the region's biggest and most damaging floods. Rain-on-snow events are projected to increase in size and frequency in the coming years, but little guidance exists for water resource managers on how to mitigate flood risk during times of rapidly changing snowpack. Their minute-by-minute decisions during winter storms can have long-lasting impacts to people, property, and water supplies.