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The Herald via Associated Press, Ian Terry
A tree is engulfed in flames during a controlled burn near a fire line outside of Okanogan, Wash., on Saturday, Aug. 22, 2015.

TWISP, Wash. — The massive cloud of smoke is expected to lift over Washington wildfires on Sunday, but as air quality improves fire behavior could become more erratic and intense, fire officials said.

"It's like a flu opening in a fireplace," explained Suzanne Flory, spokeswoman for the U.S. Forest Service and the Rocky Mountain Incident Management Team. "Smoke serves as a cap on the fire."

The Okanogan Complex of wildfires was measured at 374 square miles Sunday morning, after growing more than 100 miles larger Saturday in what fire officials said was a relatively calm fire day.

Sunday was expected to be a different story. Once the smoke lifts, humidity drops, heat rises and fires flare up.

The good news is that less smoke means restrictions on air travel will be lifted and more fire tankers can drop water and chemical retardant, Flory said.

Air quality, which has been dangerously bad, will also improve when the smoke cloud lifts, but firefighters won't be able to take a breather.

"We tell firefighters, if you see blue sky, heads up," Flory said.

Meanwhile, local officials have downgraded some evacuation notices, allowing some people to return to their homes. Thousands remain under evacuation notices.

Sarah Miller, a spokeswoman with Okanogan County Emergency Management, says residents have been warned to stay ready to leave at any time and to not drive around looking at the fires.

"People driving around are getting in the way of fire operations," Miller said.

A new firefighting mobilization center is being set up at Fairchild Air Force Base near Spokane to help fight wildfires in Washington state. The base will be the staging area for 20 large fire engines and 10 water takers and will be run by a team from San Diego, California.

The new firefighting resources come one day after the Obama administration approved Washington Gov. Jay Inslee's request for a federal emergency declaration to help firefighting efforts in the eastern part of the state.

The new fire engines are coming from Utah, Nevada, Arizona and Colorado, Inslee's office said.

Sixteen large wildfires are burning across central and eastern Washington, covering more than 920 square miles. More than 200 homes have been destroyed and more than 12,000 homes and thousands of other structures remain threatened.

More than 1,000 people were fighting just the Okanogan Complex of fires on Sunday.

Blankinship reported from Seattle.