Extreme fire weather and the historical context of the 2020 Labor Day fires

David E. Rupp, Oregon Climate Change Research Institute, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, John T. Abatzoglou, Management of Complex Systems, University of California, Merced, CA, Larry W. O'Neill, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR and Mojtaba Sadegh, Department of Civil Engineering, Boise State University, Boise, ID

Background/Question/Methods
A conflagration of wildfires in western Oregon spread rapidly during a strong easterly wind that began on Labor Day in 2020. The Labor Day fires burned more area of the Oregon Cascades (11%) than had burned in the previous 36 years combined and very likely exceeded the area burned in any single year for at least the past 120 years. The magnitude of the fires prompted several questions concerning the antecedent and concurrent meteorological conditions (i.e., the fire weather) driving the fires. How extreme and rare was the fire weather at this time, and in what respects, given fire weather is the product of a compound set of factors? Should we have been surprised, or have similar conditions given rise to very large fires in western Oregon in the not-so-distant past? Should we be concerned with such an event becoming more likely under projected climate change?
Results/Conclusions
Unusually warm conditions and severe drought with limited precipitation in the two months prior to the fires allowed for fuels to become particular combustible by early September. Downslope offshore winds during 7–9 September 2020 across the Oregon Cascades brought exceptionally strong surface winds and dry air that drove rapid rates of fire spread. While none of these individual factors were record-setting, the concurrence of these drivers created conditions unmatched in the observational record. A survey of previous very large fires in western Oregon since 1900 reveals a pattern of preceding anomalously hot and dry summers combined with an easterly wind in late summer/early fall delivered by a similar synoptic-scale weather pattern. Projected increases in the likelihood of extreme fire weather through the 21st century stems from increasing aridity during the summer and early fall fire season. Potential decreases in the frequency of easterly downslope winds are expected to be small, and insufficient to significantly counter the effect of a warmer and drier fire season, resulting in increased forest fire hazard in the Oregon Cascades.