Exploring the Origins of Snow Droughts in the Northern Sierra Nevada, California

The concept of snow drought is gaining widespread interest as the climate of snow-dominated mountain watersheds continues to change. Warm snow drought is defined as above or near average accumulated precipitation coinciding with below average snow water equivalent at a point in time. Dry snow drought is defined as below average accumulated precipitation and snow water equivalent at a point in time. We contend that such point-in-time definitions might miss important components of how snow droughts originate, persist, and terminate. Using these simple definitions and a variety of observations at monthly, daily, and hourly timescales, we explore the hydrometeorological origins of potential snow droughts in the northern Sierra Nevada from water years 1951 to 2017. We find that snow droughts can result from extreme early season precipitation, frequent rain on snow events, and low precipitation years. Late season snow droughts can follow persistent warm and dry periods with effects that depend upon elevation. Many snow droughts were characterized by lower snow fractions and frequent midwinter peak runoff events. Our findings can guide improved evaluations of historical and potential future snow droughts, particularly with regards to how impacts on water resources and mountain ecosystems may vary depending on how snow droughts originate and evolve in time.