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Decreasing Number of Rainy Days in Summer Has Increased Western Wildfire

Posted date: August 20, 2018

The number and size of large wildfires have increased dramatically in the western United States during the past three decades. Contrary to previous understanding, new research shows that significant declines in summer precipitation and lengthening dry spells during summer are major drivers of the increase in fire activity. Prior understanding was that the increase in fires was attributable mainly to warming temperatures and earlier snowmelt.

The study, “Decreasing fire season precipitation increased recent western US forest wildfire activity,” was published this week in the Proceedings of the National Academy of Sciences and was conducted by a team of scientists from the USDA Forest Service and the University of Montana.

Forest Fire Thumbail
Forest Fire Thumbail

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

“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 Rocky Mountain Research Station scientist 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 Rocky Mountain Research Station scientist and co-author of the study.

The study, funded by NASA and USDA, was conducted as part of a larger project aimed at improving wildfire danger and drought monitoring.


The Rocky Mountain Research Station (RMRS) is one of seven units within the U.S. Forest Service Research and Development. RMRS maintains 14 field laboratories throughout a 12-state territory encompassing parts of the Great Basin, Southwest, Rocky Mountains, and the Great Plains. RMRS also administers and conducts research on 14 experimental forests, ranges and watersheds and maintains long-term research databases for these areas. While anchored in the geography of the West our research is global in scale. To find out more about the RMRS go to You can also follow us on Twitter at




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Jennifer Hayes
Public Affairs Specialist