2 Dead After Heavy Rain, Rapid Snowmelt in California and Nevada Trigger Flooding, Mudslides and Dam Failures
A warmer atmosphere drives more extreme precipitation across all storm types, and warming temperatures convert snowfall to rainfall, which in turn can melt snow pack. Both factors increase the risk of flooding. In 2015, for the first time in 120 years of record keeping, the winter average minimum temperature in the Sierra Nevada was above freezing, a key threshold for maintaining snow pack. 68% of weather stations in California between 2,000' and 5,000' of elevation are recording a lower percentage of precipitation falling as snow during the winter snow season over the last several decades.
Climate conditions in California are polarizing further, amplifying the flood/drought pattern that has repeatedly visited the state over the last century. Overall basin dryness has increased, due to warming temperatures which dry out soils and amplifies drought conditions. This in turn decreases flood risk. However at the same time, as rainfall is concentrated into extreme precipitation events and as increasingly less precipitation falls as snow, flood risk increases. These trends combine to drive weather toward extremes at either end of the drought/flood spectrum.
Parts of Nevada and California continue to battle heavy rain and rapid snowmelt in the Sierra Mountains, which has led to at least two deaths attributed to widespread flooding that triggered numerous mudslides and road washouts. In Oroville, California, water opened up a massive hole in the nation's largest dam and a state of emergency was declared in Nevada after an earthen dam failed.