UC San Diego

A recently developed cutoff low catalog and a long-term precipitation dataset from 1982 to 2024 were analyzed to examine how cutoff lows contribute to annual and seasonal precipitation variability across the U.S. West Coast (USWC) compared to atmospheric rivers (ARs). Assessments were conducted for three regions: Southern California (SoCal), Northern California (NorCal), and the Pacific Northwest (PNW). On average, 5% of the total precipitation was associated with cutoff lows only, while 48% was associated with ARs only across all three regions. While the relative precipitation contributions from cutoff lows were <5% in the PNW and NorCal, the SoCal region received 9% of its total precipitation from cutoff lows, underscoring their greater importance in SoCal. The PNW and NorCal regions had their peak cutoff low precipitation contributions from May to July, while SoCal’s peak was from September to November. Cutoff lows were most frequent in different regions and months, with SoCal having the most frequent landfalling occurrences. The PNW experienced its highest average cutoff low frequencies from January to May, whereas NorCal had its highest average cutoff low frequencies from March to May, and September to November. SoCal’s highest average cutoff low frequencies occurred between September and November. While the PNW accounted for the largest total precipitation across the USWC, SoCal had the largest share of precipitation associated with cutoff lows. Precipitation from cutoff lows in SoCal accounted for ~41% of all cutoff low precipitation of the USWC, with the PNW accounting for ~26% and NorCal accounting for ~33%.