The 2017 North Bay and Southern California Fires: A Case Study

Abstract

Two extreme wind-driven wildfire events impacted California in late 2017, leading to 46 fatalities and thousands of structures lost. This study characterizes the meteorological and climatological factors that drove and enabled these wildfire events and quantifies their rarity over the observational record. Both events featured key fire-weather metrics that were unprecedented in the observational record that followed a sequence of climatic conditions that enhanced fine fuel abundance and fuel availability. The North Bay fires of October 2017 occurred coincident with strong downslope winds, with a majority of burned area occurring within the first 12 h of ignition. By contrast, the southern California fires of December 2017 occurred during the longest Santa Ana wind event on record, resulting in the largest wildfire in California's modern history. Both fire events occurred following an exceptionally wet winter that was preceded by a severe four-year drought. Fuels were further preconditioned by the warmest summer and autumn on record in northern and southern California, respectively. Finally, delayed onset of autumn precipitation allowed for critically low dead fuel moistures leading up to the wind events. Fire weather conditions were well forecast several days prior to the fire. However, the rarity of fire-weather conditions that occurred near populated regions, along with other societal factors such as limited evacuation protocols and limited wildfire preparedness in communities outside of the traditional wildland urban interface were key contributors to the widespread wildfire impacts.

Observed Impacts of Anthropogenic Climate Change on Wildfire in California

“…In summer, when fires are most frequent in California, large burned areas are promoted by the cumulative drying effects of atmospheric aridity and precipitation deficits mainly in forest ecosystems where fuel availability is not a limiting factor (Abatzoglou & Kolden, 2013;Jin et al, 2014;Keeley & Syphard, 2016;Swetnam, 1993;Swetnam & Betancourt, 1998;Westerling et al, 2003;Williams et al, 2018). In fall, many of California's most destructive fires occur in coastal shrublands and are driven by often extreme offshore downslope wind events, where synoptic conditions advect dry air masses often originating from the continental interior high desert westward and southward across topographic barriers such as the Transverse, Peninsular, and Coastal Ranges (Conil & Hall, 2006;Guzman-Morales et al, 2016;Moritz et al, 2010;Nauslar et al, 2018). The most widely studied offshore wind events, termed Santa Ana winds in southern California, increase in frequency in the fall and peak in winter Raphael, 2003).…”

Spatiotemporal prediction of wildfire size extremes with Bayesian finite sample maxima

“…, Nauslar et al. ), predictions of future extremes at a national level could inform disaster related resource allocation. The term “extreme” has multiple meanings with respect to wildfires (Tedim et al.…”

Snow Level Characteristics and Impacts of a Spring Typhoon-Originating Atmospheric River in the Sierra Nevada, USA

“…Heavy precipitation can cause streamflows to rise above baseflow, which generally benefits ecosystems. The wetting of soils and vegetation can reduce wildfire hazard when fuel moistures are near their climatologically lowest values [50]. Conversely, under favorable synoptic and mesoscale conditions, the abundant moisture associated with these storms (Figure 4a) can generate short-duration, intense precipitation (Figure 4b), which can lead to damaging post-fire debris flows [7].…”