Bootleg fire is now generating its own weather

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A towering cloud of hot air, smoke and moisture reached the heights of the aircraft and caused lightning. Wind-driven flame fronts that darted across the landscape, often jumping from bonfires. Even, possibly, a rare fire tornado.

The Bootleg Fire in southern Oregon, spurred by months of drought and a severe heatwave last month, is the largest wildfire so far this year in the United States, having already burned more than 340,000 acres, or 530 square miles, of forests and grasslands.

And while climate change is causing wildfires to be bigger and more intense, they are also one of the most extreme, so big and hot that they affect the winds and disrupt the atmosphere.

“The fire is so big and generates so much energy and heat that it’s so intense that it changes the weather,” said Marcus Kaufmann, a spokesperson for the state forest department. “The weather usually predicts what the fire will do. In this case, the fire predicts what the weather will do.”

The Bootleg Fire had been burning for two weeks, and for most of that time it exhibited one or more types of intense fires, resulting in rapid changes in wind and other conditions that caused the flames to spread rapidly in the forest canopy, igniting all trees standing at once, and the embers Volatile over long distances, quickly setting fires elsewhere.

“It’s kind of a dangerous and extreme situation,” said Chuck Redman, a forecast expert with the National Weather Service who was at the fire headquarters providing forecasts.

Fires so intense that they generate a weather of their own that confuses firefighting efforts. The intense intensity and heat can push the winds around, creating clouds and sometimes generating so-called fire tornadoes – vortices of heat, smoke and gale-force winds.

The Carr Fire disaster near Reading, California, in July 2018, was one of those fires, burning 130,000 acres, destroying more than 1,600 buildings and killing at least eight people, some attributable to a hurricane fire with winds up to 140 mph captured on video.

Many wildfires quickly grow in size, and the Bootleg Fire is no exception. In the first few days it grew by a few square miles or less, but in the later days it has grown by 80 square miles or more. Almost every day, erratic conditions forced some of the approximately 2,200 firefighting personnel to retreat to safer locations, further hampering efforts to control them. More than 75 homes and other facilities were burnt.

And Thursday evening along the northern ledge, the fire jumped over a line that had been treated with chemical inhibitors, forcing firefighters to retreat. It was just the latest example of a fire engulfing a fire barrier.

“This fire is a real challenge, and we look forward to an ongoing battle for the foreseeable future,” said Joe Heisel, Incident Commander in the Forestry Division.

It is likely to remain unpredictable.

“Fire behavior is a function of fuel, terrain, and weather,” said Craig P. Clements, director of the Wildfire Center for Interdisciplinary Research at San Jose State University. “It generally changes day by day. Sometimes minute by minute.”

Almost every day, Mr. Redman said, the fires create high currents of hot air, smoke and moisture called pyrocumulus clouds, some as high as 30,000 feet. One day, he said, they saw one of these clouds break, which can happen early in the evening when the updraft stops.

“All this mass has to recede,” he said, pushing the air on the surface outward, creating strong, gale-force winds in all directions that can spread fire. “This is not a good thing.”

Last Wednesday, though, conditions led to the formation of a larger and longer cloud called pyrocumulonimbus, which resembles the head of thunder. It’s likely to be about 45,000 feet high, said Neil LaRue, who studies wildfire behavior at the University of Nevada, Reno.

Like a head of thunder, the massive cloud triggered lightning strikes, worrying firefighters due to its ability to ignite new fires. It may also have caused precipitation.

“Some of these events are raining on themselves,” said John Bailey, a professor of forestry at Oregon State University.

Rain can be a good thing, by dampening some fuels and helping to slow a fire. But by cooling the air near the surface, precipitation can also lead to dangerous undercurrents, Dr. LaRue said.

There have also been reports of fiery whirlpools, small whirlpools of air, and flames that are common to many wildfires and are often inaccurately described as fire tornadoes. Fire eddies are small, perhaps a few tens of feet in diameter at their largest, and last from a few seconds to a few minutes.

But Dr. LaRue said there is some evidence that the Bootleg Fire may have caused an actual fire hurricane, which can reach several thousand feet in diameter, have winds exceeding 65 miles per hour, extend thousands of feet in the air and last much longer. “It seemed to be producing a very important alternation,” he said.

Fire tornadoes occur when a column of hot air rises inside a fire, attracting more air from the outside to replace it. Local terrain and differences in wind direction, often caused by the fire itself, can give spin to this flowing air, and the expansion of the air column can cause it to spin faster, like a snowboarder pulling his arms out to increase his strength. yarn.

Mr. Redman said the incident command had not received any reports of a hurricane fire. “But it is entirely possible,” he said, that someone could have been in a fire of this magnitude and severity. “When we get these extreme events, we have to monitor them.”

Other types of excessive fire behavior are more common. But the duration of the Bootleg Fire’s extreme behavior has stunned some of those who fight it.

“It’s day in and day out of this extreme behavior and exponential growth,” said Mr. Kaufman. “And you can’t really fight fire under these conditions. It’s too dangerous.”

The root cause of most extreme behavior is the massive amount of heat that the fire pumps out.

The amount of heat is related to the drying out of the fuel – trees and other plants, both living and dead. The fuel in southern Oregon, as well as much of the west, is extremely dry, as a result of the severe drought that has affected much of the region.

Dr. Clements likened him to a campfire. “You want dry rugs and logs to make a fire,” he said. “Same in a forest fire. That’s why we’ve been watching the drought.”

If the vegetation is wet, some of the energy from the combustion is used to evaporate its moisture. If there is no moisture to evaporate, the fire burns hotter. “More heat is released,” he said. “The fire is bigger.”

Oregon was also hit in late June by a heatwave, when temperatures were broken in some places by as much as 9 degrees Fahrenheit. This caused the vegetation cover to dry out even more. In southern Oregon, the fuel was as dry as it was at the end of summer in a more normal year.

“We had a lot of fuel that was ready to burn,” Dr. Bailey said.

What would help end the extreme behavior, and ultimately the fire itself, is a good, widespread rainfall. But it does not appear that this is imminent.

“We’re not seeing any major relief for the next week at least,” Redman said. “But I don’t think we could get any worse.”

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