Autumn precipitation: the competition with Santa Ana winds in determining fire outcomes in southern California

Background California's South Coast has experienced peak burned area in autumn. Following typically dry, warm summers, precipitation events and Santa Ana winds (SAWs) each occur with increasing frequency from autumn to winter and may affect fire outcomes. Aims We investigate historical records to understand how these counteracting influences have affected fires. Methods We defined autumn precipitation onset as the first 3 days when precipitation >= 8.5 mm, and assessed how onset timing and SAWs were associated with frequency of >= 100 ha fires and area burned during 1948-2018. Key results Timing of autumn precipitation onset had negligible trend but varied considerably from year to year. A total of 90% of area burned in autumn through winter occurred from fires started before onset. Early onset autumns experienced considerably fewer fires and area burned than late onset autumns. SAWs were involved in many of the large fires before onset and nearly all of the lesser number after onset. Conclusions Risk of large fires is reduced after autumn precipitation onset, but may resurge during SAWs, which provide high risk weather required to generate a large fire. Implications During autumn before onset, and particularly during late onset autumns, high levels of preparation and vigilance are needed to avoid great fire impacts.