The ingredients for a severe wildfire season

The ingredients for a severe wildfire season
Although hot temperatures and poor forest health contribute to wildfire intensity, one forest biology professor says summer rainfall can be the deciding factor in how severe a wildfire is, as well as the overall season. Photo:

While Washington’s wildfire season is far from over, so far it has proved to be considerably less severe than in recent years. As of this week, there have been a total of 999 wildfires that have burned 128,969 acres – 15,521 acres of that on land managed by the state Department of Natural Resources (DNR). Last year, more than three times that number of acres burned in Washington (438,868 acres). The 2015 wildfire season was the worst in state history, with 1,500 wildfires that burned more than 1 million acres and included one of the largest complex fires in Washington.

Although forestry experts, conservationists and state officials have emphasized the connection between poor forestland health in eastern Washington and wildfire severity, other factors play a critical role in determining how severe or mild the overall wildfire season will be. Weather elements such as hot temperatures, low humidity and topography can affect how fast and large a wildfire grows. Another contributing factor has been the improved initial attacks since 2015 by DNR and other firefighting agencies that have kept 95 percent of wildfires at 10 acres or less; roughly 98 percent of wildfires are kept below 100 acres.

Yet University of Washington Forest Biology Professor David Peterson says the amount of rainfall that occurs during the months of July and August is also a key factor. Peterson is also a former senior research scientist for the U.S. Forest Service.

He told Lens that the main ingredients for a severe wildfire are lots of small-diameter fuels, a long dry spell and ignition usually caused by a lightning strike.

“Clearly if you have high winds that will help to drive the fire,” he said. “But even in the absence of wind in mountainous topography, once you have a fire burning, the heat of the earth and atmosphere creates its own wind.”

Though 2015’s winter had low snowpack levels contributing to a water drought later in the year, he said “it does not matter at all what happens in the winter. We can have a very wet winter. It’s what happens in July or August. No one can forecast wildfire seasons months ahead.”

The summer in particular has been marked by unusually high rainfall for this time of year. According to the Northwest Interagency Coordination Center (NWCC), parts of central and eastern Washington received 300-400 percent above average precipitation. Not only did it help contain the Williams Flat Fire in northcentral Washington, but the rain kept those light fuels wet during thunderstorms. The effects are reflected in NWCC fuel load data of energy release component (ERC), which indicates how hot a fire can burn. In July, the ERC was below average, with a steep decrease in early August.

“That summer precipitation is critical, even a little bit,” Peterson said. “You have the initial precipitation which moistened fuels, but it also keeps the humidity up longer at night and longer in the morning.”

In July 2015, the city of Winthrop received only .08 inch of rainfall compared to a .75 inch average. This July, it received .9 inch of rainfall. With an average rainfall of .35 inches in July, the city of Chelan received no precipitation that month in 2015, but .59 inch this July.

Although it doesn’t seem like much, Peterson said “that periodic rain, an inch a month, is often enough” to either prevent wildfires from starting or reduce their severity.

He added: “The objective is not to stop fires…but to reduce the intensity….so we can allow them to burn as they did for thousands of years in some of these landscapes without canopy fires. Or, it gives our firefighters an opportunity to put them out.”


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