Compound Extremes Drive the Western Oregon Wildfires of September 2020

Several very large high-impact fires burned nearly 4,000 km(2) of mesic forests in western Oregon during September 7-9, 2020. While infrequent, very large high-severity fires have occurred historically in western Oregon, the extreme nature of this event warrants analyses of climate and meteorological drivers. A strong blocking pattern led to an intrusion of dry air and strong downslope east winds in the Oregon Cascades following a warm-dry 60-day period that promoted widespread fuel flammability. Viewed independently, both the downslope east winds and fuel dryness were extreme, but not unprecedented. However, the concurrence of these drivers resulted in compound extremes and impacts unmatched in the observational record. We additionally find that most large wildfires in western Oregon since 1900 have similarly coincided with warm-dry summers during at least moderate east wind events. These results reinforce the importance of incorporating a multivariate lens for compound extremes in assessing wildfire hazard risk.

Automated summary

The large high-impact fires in western Oregon in September 2020 were caused by a combination of dry air intrusion and strong east winds. These compound extremes were unmatched in the observational record, highlighting the importance of assessing wildfire hazard risk with a multivariate lens.