Study: Decreasing Number of Rainy Days in Summer Has Increased Western Wildfire

August 20, 2018

MISSOULA – The number and size of large wildfires has increased dramatically in the western United States during the past three decades. New research shows that significant declines in summer precipitation and lengthening dry spells during summer are major drivers of the increase in fire activity. This is contrary to previous understanding that the increase is attributable only to warming temperatures and earlier snowmelt.

The research was conducted by a team of scientists from the USDA Forest Service and the University of Montana, funded by NASA and the USDA, and published this week in the Proceedings of the National Academy of Sciences. The paper is online at

The research team contrasted the effects of snowmelt timing, warming summer temperatures and variations in the volume and distribution of summer precipitation on wildfire area burned. They found that summer precipitation totals and the duration of dry spells were the strongest controls on forest wildfire area burned.

“Summer dry periods are tightly coupled to how warm and dry the air is during the fire season,” said Zack Holden, USDA Forest Service scientist and lead author of the study. “Longer windows without rain lead to more surface heating which dries out woody fuels.”

“The maps of declining precipitation help us think about patterns of future drought, which can help us focus work near communities likely to experience continuing declines,” said Charlie Luce, USDA Forest Service research hydrologist and co-author of the study.

“This new information can help us better monitor changing conditions before the fire season to ensure that areas are prepared for increased wildfire potential. Further, it may improve our ability to predict fire season severity,” said Matt Jolly, USDA Forest Service research ecologist and co-author of the study.

The study was conducted as part of a larger project aimed at improving wildfire danger and drought monitoring.

UM hydrology and hydrologic modeling Associate Professor Marco Maneta was also a co-author.

“Decreases in precipitation and the increasing length of dry spells during the summer – a time when crop water demand in the arid west are highest – is not only exacerbating wildfires but could also have serious implications for western agriculture, especially in states highly reliant on rainfed crops,” Maneta said.


Contact: Zack Holden, scientist, U.S. Forest Service, 406-396-3151,