The Extraordinary Period of Atmospheric River Activity During December 2022 – January 2023 in California: Impacts and Climatological Perspectives

Between 26 December 2022 and 17 January 2023, a series of nine atmospheric rivers (ARs) made landfall in rapid succession over California. Seven of the nine ARs were classified as an AR 3 on the CW3E AR Scale, and one was classified as an AR 4. The extended period of strong AR activity produced extreme precipitation across much of the state, resulting in widespread flooding and landslides. At least 21 deaths and more than 1,400 rescues were reported. An analysis by Moody’s Risk Management Solutions suggests that total economic losses from these storms may have exceeded $5 billion. A maximum 23-day total rainfall of 47.74 inches was observed at Honeydew, CA, and 240 inches of total snowfall were recorded at Mammoth Mountain. All nine landfalling ARs were associated with mesoscale features that can modulate the duration, intensity, and spatial distribution of precipitation, such as mesoscale frontal waves, narrow cold-frontal rainbands, and Sierra barrier jets.

From a climatological perspective, the magnitude of AR activity and precipitation during this 23-day period was exceptional. While it is not uncommon for AR families to impact the U.S. West Coast, the sequence of ARs between 26 December 2022 and 17 January 2023 was the longest-duration AR family in the Scripps Institution of Oceanography’s (SIO) 70-year catalog of landfalling ARs. Time-integrated water vapor transport in Central California exceeded that of any previous 23-day period based on the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5), surpassing other notable periods of AR activity in February 1986 and January/February 2017. Many locations in Central California and coastal Southern California received more than 50% of their normal annual precipitation in less than four weeks. Persistently high streamflow levels were observed throughout much of California during this period, with cumulative 23-day streamflow exceeding the historical median annual streamflow on sections of the Cosumnes, Pajaro, and Salinas Rivers. Heavy snowfall at higher elevations set the stage for record-breaking seasonal snowpack in the Central and Southern Sierra Nevada. Despite the numerous hazardous impacts, these ARs played a crucial role in eradicating a severe multi-year drought and significantly improving California’s water supply outlook.