March 24, 2023

A towering cloud of hot air, smoke, and moisture that reached airplane heights and caused lightning. Wind-driven flame fronts that have stamped across the landscape and often jump over firebreaks. Maybe even a rare fire tornado.

The Southern Oregon Bootleg Fire, sparked by months of drought and last month’s scorching heatwave, is the largest wildfire to date this year in the United States, having already burned more than 340,000 acres, or 530 square miles of forest and grassland.

And at a time when climate change is leading to larger and more intense forest fires, it is also one of the most extreme, so big and hot that it affects winds and otherwise disrupts the atmosphere.

“The fire is so big and generates so much energy and extreme heat that it changes the weather,” said Marcus Kauffman, spokesman for the State Forestry Office. “Usually the weather predicts what the fire will do. In this case the fire predicts the weather. “

The Bootleg Fire has been burning for two weeks, and most of the time it has exhibited one or more forms of extreme fire behavior, resulting in rapid wind changes and other conditions that have caused flames to spread rapidly through the forest canopy, entirely ignites trees at once and embers over long distances that quickly ignite point fires elsewhere.

“It’s an extreme, dangerous situation,” said Chuck Redman, a forecaster with the National Weather Service who has been to the fire department headquarters making forecasts.

Fires so extreme that they create their own weather-related extinguishing efforts. The intensity and extreme heat can force the wind to bypass them, creating clouds, and sometimes even creating so-called fire tornadoes – swirling eddies of heat, smoke, and high winds.

The catastrophic fire in Carr near Redding, California, in July 2018 was one of those fires that covered 130,000 acres, destroyed more than 1,600 buildings and resulted in the deaths of at least eight people, some of whom resulted in a fire tornado with winds of up to to 140 mph recorded on video.

Many forest fires grow in size quickly and the bootleg fire is no exception. In the first few days it grew a few square miles or less, but in the last few days it grew 80 square miles or more. And almost every day, unpredictable conditions are forcing some of the nearly 2,200 firefighters to retreat to safer locations, further hampering efforts to get the fire department under control. More than 75 houses and other buildings burned.

On Thursday night, the fire on the northern edge jumped over a pipe treated with chemical protective agents and forced firefighters to withdraw. It was just the latest example of the fire that rolled over a firebreak.

“This fire is a real challenge and we aim to have an ongoing battle for the foreseeable future,” said Joe Hessel, the forestry operations commander.

And it will likely continue to be unpredictable.

“Fire behavior is a function of fuels, topography, and weather,” said Craig B. Clements, director of the Wildfire Interdisciplinary Research Center at San Jose State University. “That generally changes from day to day. Sometimes minute by minute. “

Mr Redman said the fire produced high currents of hot air, smoke and moisture called pyrocumulus clouds almost every day, some of which can reach 30,000 feet. One day, he said, they saw one of these clouds collapse, which can happen in the early evening when the updraft stops.

“The whole mass has to come down again,” he said, which pushes the air on the surface outwards and creates strong, gusty winds in all directions that can spread a fire. “This is not good.”

However, last Wednesday, conditions resulted in the formation of a larger, higher cloud called pyrocumulonimbus that resembles a thundercloud. It likely reached an altitude of about 45,000 feet, said Neil Lareau, who studies forest fire behavior at the University of Nevada at Reno.

Like a thunderstorm, the huge cloud produced lightning strikes that worried firefighters because they have the potential to start new fires. It may also have brought precipitation with it.

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

Rain can be a good thing by dampening some of the fuels and slowing down the fire. However, by cooling the air closer to the surface, rain can also create dangerous downdrafts, said Dr. Lareau.

There have also been reports of vortices of fire, small vortices of air and flames that are common in many forest fires and are often incorrectly described as fire tornadoes. Vortices of fire are small, perhaps a few tens of feet in diameter at their largest, and last a few seconds to a few minutes.

But dr. Lareau said there is some evidence that the Bootleg Fire may have created an actual fire tornado that can be several thousand feet in diameter, has wind speeds in excess of 100 miles per hour, and extends and sustains thousands of feet in the air much longer. “It looks like it created quite a significant rotation,” he said.

Fire tornadoes occur when a cloud of hot air rises inside a fire, drawing in more outside air to replace it. Local topography and differences in wind direction, often caused by the fire itself, can give this incoming air a twist, and stretching the column of air can cause it to spin faster, like a figure skater pulling her arms around to enlarge them. rotate.

Mr. Redman said the task force had received no reports of a fire tornado. “But it is entirely possible” to perform in such a large and intense fire, he said. “When we get these extreme events, we have to be careful.”

Other types of extreme fire behavior are more common. But the duration of the extreme behavior in bootleg fire has baffled some of those who fight it.

“This extreme behavior and explosive growth every day,” said Mr. Kauffman. “And you can’t really fight fire under these conditions. It is too dangerous.”

The main cause of most extreme behaviors is the enormous amount of heat that the fire emits.

The amount of heat depends on the dryness of the fuel – trees and other vegetation, both dead and alive. And fuels in southern Oregon, as well as most of the west, are extremely dry due to the severe drought that hit most of the region.

Dr. Clements compared it to a campfire. “You want the driest tinder and logs to start the fire,” he said. “Same with a forest fire. That is why we are watching the drought. “

When the vegetation is damp, some of the combustion energy is used to evaporate its moisture. If moisture doesn’t evaporate, the fire will burn hotter. “More heat is released,” he said. “The flames are bigger.”

Oregon was also hit by an extreme heat wave in late June, when record temperatures were broken by as much as 9 degrees Fahrenheit in some places. That dried out the vegetation even more. In southern Oregon, the fuel was as dry as a normal year by the end of summer.

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

What would help end the extreme behavior, and ultimately the fire itself, is good, widespread rain. But that doesn’t seem in sight.

“We don’t see any significant relief for at least the next week,” Redman said. “But I don’t think we can get any worse.”