Snowpack Contributions and Temperature Characterization of Landfalling Atmospheric Rivers in the Western Cordillera of the United States

Atmospheric rivers (ARs) are regions of high water vapor transport in the lower atmosphere. When these air masses encounter mountain ranges, they can produce significantly enhanced orographic precipitation. AR events may substantially influence seasonal mountain snow, an important ecologic and economic resource for Washington, Oregon, and California. To better understand how ARs affect the montane snowpack of these U.S. West Coast states, we used 33 years of Snow Telemetry and Topography Weather data to examine AR and non-AR storm day temperatures and impacts on snow water equivalent (SWE). We found mean daily minimum temperatures of AR storm days to be 1 to 4 degrees C warmer and mean daily increases in SWE to be 23% to 57% higher than non-AR storm days. AR storm days have contributed an average of 23% of effective snowfall to seasonal SWE in the California Sierra Nevada and 34% of effective snowfall to seasonal SWE in the Washington and Oregon Cascades. Plain Language Summary Atmospheric rivers (ARs), or "rivers in the sky," are areas where large amounts of water vapor are quickly transported through the lower atmosphere. When these air masses are pushed up by mountain ranges, precipitation can increase dramatically. AR-related precipitation can substantially affect seasonal mountain snow, which has strong economic and ecological roles in Washington, Oregon, and California. To better understand how ARs affect mountain snow in these states, we used 33 years of ground-based weather and snow data to examine average temperatures and snow accumulation and melt on AR and non-AR storm days. Compared to non-AR storm days, daily minimum temperatures of AR storm days were 1 to 4 degrees C warmer and daily increases in snow accumulation were 23% to 57% higher. These very warm and very wet storm days play a smaller role in California than previously thought. On average, AR days are responsible for 23% of seasonal snowfall in the California Sierra Nevada and 34% of seasonal snowfall in the Washington and Oregon Cascades.