The blazes that raced across western Oregon this week could be the most unexpected element in a fire season that’s full of surprises: Not just more wildfires, but wildfires in places that don’t usually burn.
The forests between Eugene and Portland haven’t experienced fires this severe in decades, experts say. What’s different this time is that exceptionally dry conditions, combined with unusually strong and hot east winds, have caused wildfires to spiral out of control, threatening neighborhoods that didn’t seem vulnerable until now.
“We’re seeing fires in places that we don’t normally see fires,” said Crystal A. Kolden, a professor of fire science at the University of California, Merced. “Normally it’s far too wet to burn.”
The fires in Oregon, which have led to the evacuation of hundreds of thousands of people and are approaching the Portland suburbs, stand out from what has already been an extraordinary fire season in the West, where global warming, land-use changes and fire management practices have combined to create a hellish mix of smoldering forests, charred homes and choking air.
Before this week, Oregon was grappling with a much more contained problem, a series of smaller fires on both sides of the Cascade Range, which divides the state between east and west.
Fires are common in the east, which is normally dry, according to Philip Mote, a climate scientist at Oregon State University. In some areas of eastern Oregon the “return period,” or length of time between major fires, is as little as 20 years, he said.
But the western slope of the Cascades, which catches most of the moisture that blows in from the Pacific Ocean, is normally wetter. “Out here, the return period can be hundreds of years,” he said.
That protective moisture has faded, in large part because climate change has altered precipitation and temperature patterns.
Tim Brown, director of the Western Regional Climate Center at the Desert Research Institute in Reno, Nev., said the extreme warmth had caused vegetation to become exceptionally dry and to burn more readily. Temperature, humidity, wind and solar radiation combine to dry out brush and are the key elements for fire. “We call it evaporative demand,” he said. And in recent weeks, he added, “the west Cascades have been really dry from the evaporative demand.”
Those dry conditions were most likely exacerbated by climate change, according to Meg Krawchuk, a professor at Oregon State’s College of Forestry. And they had the effect of “teeing up the landscape” for a wildfire, she said.
The critical moment came Monday and Tuesday, when a windstorm carried hot air from the high desert in the eastern part of Oregon over the mountains, rapidly spreading the fires in the more populated western part of the state, according to Josh Clark, fire meteorologist at the Washington State Department of Natural Resources.
Those winds were the strongest the state has seen in at least 30 years, Mr. Clark said. And when they crossed the mountains, the winds raced down through river canyons, which compressed the air, warming it further and pushing it westward like a bellows.
As those fires raced west, they met unusually dry conditions, said Dr. Kolden, which in turn allowed the fires that were already burning to spread rapidly. “The fire’s able to move very quickly and just explode down these canyons,” she said.
The fires now threatening Oregon’s cities and towns could be worse than anything in that part of the state in decades, said Cassandra Moseley, chief research officer at the University of Oregon and a professor at its Institute for a Sustainable Environment.
The Tillamook Burn, a series of fires that began in 1933 and destroyed hundreds of thousands of acres, was probably as bad as this week’s fires, Dr. Moseley said. It’s hard to know for sure, she said, because “no one’s alive to tell the tale.”
And what’s different this time, Dr. Moseley said, is that far fewer people lived in those areas 90 years ago. “Tillamook didn’t have people in it,” she said. By comparison, this week’s fires seem likely to cause large numbers of casualties.
Already, several mountain communities had been destroyed by flames that roared though the surrounding forests. State officials received reports of dozens of missing people. And as some of the largest blazes neared Portland’s southern suburbs, the authorities warned residents thinking of staying behind in some communities that there would be no firefighters to protect them.
The lesson of this week is that the state must now prepare for more of the same, said Dr. Mote, the Oregon State climate scientist, who recalled that extreme warmth had also led to a record low snowpack in 2015.
“This situation of large fires, and that low snow year — these are both things that I and my colleagues who’ve studied climate change in Oregon for 20 years have been saying would happen eventually,” he said. “And now they’re happening.”