Freezing Levels and Atmospheric Rivers over the California Feather River Watershed

An atmospheric river (AR) is a long, narrow, and transient corridor of horizontal water vapor flux that primarily affects the west coasts of continents, including northern California. The freezing level during an AR event can affect its impacts; a low (high) freezing level (Z0°C) will cause snow (rain). The Feather River Watershed in northern California was examined to learn more about ARs, freezing levels, and related impacts at and downstream of Lake Oroville, California’s second largest reservoir. AR data from the Rutz et al. (2014) catalog and freezing level data from NASA MERRA-2 reanalysis were used to analyze AR characteristics, and data from the California Data Exchange Center (CDEC) were used for inflow and precipitation analysis at Oroville Dam (ORO) and at the Brush Creek (BRS) and Quincy (QCY) stations. During WY1981-2017 there were 978 periods with AR conditions (Integrated vapor transport magnitude ≥ 250 kg/m/s) longer than 3-hr in duration with the highest frequency in January with 153 ARs. In the Feather River Watershed’s cold season, 40% of the inflow at ORO is caused by the top 10% of periods with AR conditions, and 28% of the inflow is caused by the top 5% of periods. This demonstrates the need for accurate AR and freezing level forecasts to better prepare for reservoir levels and potential flooding hazards downstream. Top 10 Warm and Top 10 Cold events from WY1981-2017 were identified by average freezing level per event and by precipitation greater than 1 inch in order to focus on extreme high and extreme low events and their impacts. By this methodology, warm events had an average duration of 39.6 hours while cold events only had an average duration of 16.5 hours. Warm events had roughly twice the precipitation amounts, and also had higher average IVT magnitude. These extreme high and low freezing level events were chosen to focus on the difference in impacts between high elevation snow events and fully rain events, as well as the different synoptic and mesoscale patterns.