WENATCHEE —

Among the pine-draped hills southwest of Wenatchee on Thursday the state's two biggest wildfires threatened to merge into an inferno larger than the city of Seattle.

Fire officials spent the day warily eying the spreading flames east of Highway 97 near Blewett Pass, not sure whether a merger might prove a welcome step toward finally wrestling these blazes under control — or lead to a far more volatile conflagration.

But after firefighters scrambled to protect more than 600 at-risk homes in Chelan and Kittitas counties, it was becoming clearer by early evening that the merger was coming whether fire officials wanted it or not.

"It's just one of those things that's probably going to happen," said fire spokeswoman Connie Mehmel. "It's really to the point where we need to back off a bit. We don't want to be in the middle of those two fires that are trying to come together."

The Table Mountain Complex and Wenatchee Complex fires had burned across more than 70,000 acres by early evening in what officials called the largest fire event in years.

As of Thursday night, no lives nor homes had been lost, but hundreds have been evacuated and smoky air continues to flood the entire region, raising public-health concerns and grounding aircraft, complicating life and the firefighting effort.

"The smoke seems to be getting thicker and thicker," said another fire spokesman, Alan Hoffmeister.

The rapidly spreading flames raged a full month after the typical peak of fire season, in a year that has seen the fewest fires nationwide in the past decade. But those fires have devastated nearly 8.5 million acres, more than in all but one of the past 10 years.

"Typically, our fire danger indices spike on Aug. 19," said Jason Loomis, a fire analyst with the Northwest Interagency Coordination Center in Portland. "But we didn't see that this year. We've just had this relentless, persistent pattern of hot, dry weather."

Rex Reed, incident commander on the Table Mountain Complex fires, said a fire this big in September is unprecedented in his career, which began in 1974. And while the behavior of the flames hasn't been extreme, they have been relentless.

"Typically our fires' burn period is six to seven hours a day, and then it moderates for the remaining 17 or 18 hours," Loomis said. "The last few days it's been gaining ground 24 hours a day, not even settling down at night."

Meanwhile, the smoke-filled valleys make seeing new blazes difficult, so by the time firefighters arrive to put out spot fires, the blazes have grown substantially.

The blazes started on Sept. 9, when a huge lightning storm ignited 110 fires.

While smaller fires continue to burn from the Methow Valley to Mount Adams, the Chelan and Kittitas blazes came together into two main groups. The Wenatchee Complex neared 42,000 acres Thursday evening and was 24 percent contained, while the Table Mountain Complex topped 30,000 acres.

Both exploded overnight Wednesday, and by Thursday it had become clear they were headed toward one another.

"When they start to get close, they can start sucking each other in, building the intensity up," said John Segar, fire-management branch chief for the U.S. Fish and Wildlife Service in Boise, Idaho.

In the worst case, merging fires can feed off each other's energy and burn hotter and send flames shooting higher into the trees where they spit out more embers and create more spot fires, causing the new larger blaze to gobble up ground even faster.

Alternatively, the two approaching fires may consume all available fuel and help snuff each other out while weary firefighters get to focus on scratching out a single fire break instead of two.

"From a strategic standpoint, it will make things easier and more predictable," said Mehmel.

The Table Mountain Complex was the more concerning during part of the day Thursday, as it appeared to lurch toward hundreds of homes including those in the small Kittitas County community of Liberty.

But the latest evacuations were farther north, where residents near the Mission Ridge ski area were urged to leave Wednesday night after reports of "raining burning ashes," said Hoffmeister.

One of those evacuees, 45-year-old homemaker Michelle Shermer, said she and her five children had been preparing for the possibility of leaving for a week.

When the call came, Shermer said, they grabbed some keepsakes — including an old dollhouse — and left.

In 13 years of living up on Forrest Ridge, the family had never before had to evacuate, Shermer said.

"We knew it would happen someday," said Shermer, sitting in a Wenatchee Wendy's parking lot and wearing a mask to protect herself from the ashy air. "You live with it up here. I'm glad we were prepared."

Many residents were wearing similar masks Thursday, including 7-year-old Emily Jimenez.

Emily couldn't help playing with the mask as she left Lewis and Clark Elementary School at around 3 p.m.

Her uncle, Moses Verduzco, said the girl would be wearing a mask for the next two weeks.

"You better believe it," the 22-year-old said. "Better than getting all kinds of diseases from the smoke."

Craig Welch: 206-464-2093 or cwelch@seattletimes.com. On Twitter @craigawelch.

Information in this article, originally published Sept. 20, 2012, was corrected Sept. 21, 2012. A previous version of this story incorrectly listed Rex Reed as the incident commander for the Wenatchee Complex fires. He is the incident commander for the nearby Table Mountain Complex fires.