On the causes of the summer 2015 Eastern Washington wildfires

Abstract

In the summer of 2015, Eastern Washington State (east of 122°W) experienced over 2000 fires that burned a record amount of area, saw the deaths of three firefighters, and caused hazardous air quality throughout much of the region. The area burned during the 2015 fire season (∼471 000 hectares) was over three times that of the next largest year in the previous 32 years. We examine Eastern Washington’s 2015 fire season in the context of the historical fire record, which is available from satellite remote sensing observations for the past 32 years. We explore the relationship between fire activity and physical climatic factors, including temperature and precipitation, fuel characteristics, such as dry fuel moisture, historical land cover change, forest health, and fire behavior, such as ignition and propagation. Summer 2015 was anomalously warm in Eastern Washington—nearly 1 °C warmer than the previous record warm year and 1.4 °C warmer than the 32-year climatology. It followed a near-record warm winter with anomalously low spring snowpack, whereas winter precipitation in 2014–2015 was near normal. We find that the extreme 2015 fire year was not attributable to any one physical factor, but rather was related to a combination of anomalously dry and warm summer conditions, a lightning storm in mid-August, early propagation and growth through grasslands, and high grassland fuel loadings caused by anomalous growth in the preceding late winter and spring. At the local level, prediction of the anomalous 2015 burned area is not associated with any single variable; however, the combination of conditions, especially climate and fuel loads, are likely to recur in the future.