Downslope Wind‐Driven Fires in the Western United States

• Published in: Earth's future Vol. 11; no. 5
• Main Authors: Abatzoglou, J. T., Kolden, C. A., Williams, A. P., Sadegh, M., Balch, J. K., Hall, A.
• Format: Journal Article
• Language: English
• Published: Bognor Regis John Wiley & Sons, Inc 01.05.2023; Wiley
• Subjects: Climate change; disasters; Downslope winds; Fire hazards; Fire prevention; Forest & brush fires; Fuels; Infrastructure; meteorology; Precipitation; Seasonal variations; Summer; Topography; Weather; wildfire; Wildfires; Wind; United States > US; California
• ISSN: 2328-4277; 2328-4277
• DOI: 10.1029/2022EF003471

Downslope wind‐driven fires have resulted in many of the wildfire disasters in the western United States and represent a unique hazard to infrastructure and human life. We analyze the co‐occurrence of wildfires and downslope winds across the western United States (US) during 1992–2020. Downslope wind‐driven fires accounted for 13.4% of the wildfires and 11.9% of the burned area in the western US yet accounted for the majority of local burned area in portions of southern California, central Washington, and the front range of the Rockies. These fires were predominantly ignited by humans, occurred closer to population centers, and resulted in outsized impacts on human lives and infrastructure. Since 1999, downslope wind‐driven fires have accounted for 60.1% of structures and 52.4% of human lives lost in wildfires in the western US. Downslope wind‐driven fires occurred under anomalously dry fuels and exhibited a seasonality distinct from other fires—occurring primarily in the spring and fall. Over 1992–2020, we document a 25% increase in the annual number of downslope wind‐driven fires and a 140% increase in their respective annual burned area, which partially reflects trends toward drier fuels. These results advance our understanding of the importance of downslope winds in driving disastrous wildfires that threaten populated regions adjacent to mountain ranges in the western US. The unique characteristics of downslope wind‐driven fires require increased fire prevention and adaptation strategies to minimize losses and incorporation of changing human‐ignitions, fuel availability and dryness, and downslope wind occurrence to elucidate future fire risk. Plain Language Summary Downslope mountain winds bring locally strong winds along with dry and often warm air to downwind slopes and are a critical fire weather pattern when such winds co‐occur with receptive fuels. Fires associated with downslope winds were primarily ignited by humans, exhibited distinct peaks in the shoulder seasons across the western United States, and had geographic hotspots in the front range of the Rockies, southwestern California, and eastern slope of the Washington Cascades. While wildfires typically burn upslope and away from populated regions, downslope wind events allow fires to rapidly spread downhill and often toward human settlements. Fires coincident with downslope winds accounted for most of the cumulative losses in both human lives and structures during 1999–2020. While winds are a key driver of these fires, large downslope wind‐driven fires preferentially occurred with anomalously dry fuels, suggesting that conditions leading up to ignitions coincident with downslope winds play an important role in enabling large fires. Lastly, we found an increase in downslope wind‐driven fires and their burned area over the past three decades. Such changes in downslope fire activity partially reflect a drying‐driven extension of the fire weather season into the spring and fall when downslope winds are more common. Key Points Downslope wind‐driven fires accounted for 13.4% of fire occurrence and 11.9% of total burned area in the western United States during 1992–2020 Most structure losses and fatalities in fires 1999–2020 were during downslope winds Downslope wind‐driven fires primarily occurred in the spring and fall, coincident with anomalously dry fuels