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

Abstract

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.

Downslope Wind‐Driven Fires in the Western United States

“…Here, we show anomalously drier fuels leading up to the ignition for downslope wind fires (e.g., above normal ERC; Figure 5a; Figure S4a in Supporting Information S1). This is particularly evident for large downslope wind fires, and has been observed for many significant offshore downslope wind‐driven fires in Oregon and California (Cayan et al., 2022; Hawkins et al., 2022). ERC is a build‐up index of fuel dryness that reflects precipitation, humidity, and temperature from the previous several weeks.…”

“…This is due to the tendency of downslope winds to occur outside of the core summer months in the western US when fuel aridity often reaches its apex. Overlapping periods of dry fuels in late‐summer and autumn (prior to the arrival of significant precipitation) and the seasonal ramp‐up of downslope winds comprises an important vulnerability window, when the ignitions can rapidly escape initial suppression and become large and potentially destructive fires (Cayan et al., 2022; Goss et al., 2020). As with previous studies, our results show large fires tended to occur with more anomalous fire weather (e.g., ERC, BI, VS) and drought (e.g., PDSI) compared to smaller fires (Abatzoglou et al., 2018; Gutierrez et al., 2022; Juang et al., 2022)—although the effects were similarly found for non‐downslope wind fires.…”

Subseasonal Prediction of Impactful California Winter Weather in a Hybrid Dynamical‐Statistical Framework

“…Predictability of extreme precipitation in a probabilistic sense could also be explored. This is especially important as both extreme winter precipitation and early winter wildfires may become more common (Cayan et al, 2022;Gershunov et al, 2019), raising the possibility of compound extreme events such as short-duration high-intensity rainfall, which can cause devastating post-fire debris flows (Oakley et al, 2017(Oakley et al, , 2018a and landslides (Oakley et al, 2018b;Rengers et al, 2020), rain-on-snow flooding (Haleakala et al, 2023), as well as other precipitation patterns driving mass movements such as avalanches (Hatchett et al, 2017). Improving lead time to prepare for these types of events and likelihood of occurrence is crucial to prevent loss of life and mitigate damage to property (Oakley et al, 2023).…”

“…Extremes of California's winter weather variability can include heavy multiday precipitation from Pacific storms associated with atmospheric rivers (ARs) or dry offshore downslope winds blowing from the elevated continental interior. ARs cause most of the region's floods (Corringham et al., 2019; Dettinger, 2013; Ralph et al., 2006, 2011) while downslope winds are often associated with coastal heat waves as well as wildfire and smoke impacts (Abatzoglou, 2013; Aguilera et al., 2021; Cayan et al., 2022; Gershunov et al., 2021; Guzman‐Morales et al., 2016; Hughes & Hall, 2010). Winter heat waves and dry spells accelerate mountain snowmelt (Hatchett et al., 2023), exacerbate drought and endanger human health, particularly along the densely populated coast (Gershunov et al., 2021; Schwarz et al., 2020).…”

Drivers of California’s changing wildfires: a state-of-the-knowledge synthesis

“…Higher temperatures, compressed precipitation season, and earlier onset of snowmelt result in drier live and dead vegetation, which contributes to increased flammability of large portions of the landscape for longer periods of time (Westerling et al 2006;Miller et al 2011;Jolly et al 2015;Kitzberger et al 2017;Ma et al 2021), and exacerbates summer fuel-dominated wildfires (Williams et al 2019). Furthermore, the effective temporal compression of California's rainy season, combined with highintensity weather events such as Santa Ana winds, directly contributes to increasing wildfire risk (Westerling et al 2004;Swain 2021;Cayan et al 2022). Increasing temperatures and decreasing precipitation in autumn are linked to increases in extreme fire weather, enhancing the risk of large autumn wildfires (Williams et al 2019;Goss et al 2020).…”