The blaze sparked Sunday afternoon was still burning out of control 12 hours later, said Rich Magnussen, a spokesman for the Chelan County Emergency Management office.
PHOTOS: Sleepy Hollow Fire
At least a thousand people have left their homes in Wenatchee, about 120 miles east of Seattle.
"We've got hundreds of homes under evacuation notices," he said.
Magnussen said the evacuations were mainly in the north end of town and included a Walmart store.
So far, at least a dozen structures -- mainly houses -- have been destroyed.
"It's probably going to turn out to be more than that," he said.
Firefighters from around the region were on the scene with more on the way.
"Hundreds of them [firefighters], and fire resources from all over eastern Washington," Magnussen said.
Sunday's temperatures of at least 108 degrees, tinder dry brush and strong winds helped fuel the fire.
"The wind really picked up in the afternoon," he said, adding Monday's forecast calls for more of the same.
Reports Sunday night said the fire had burned 2.6 square miles of territory. Magnussen said it had grown since then but officials won't know by how much until daylight.
There have been no reports of serious injury, but several homes have been destroyed by the flames.
"We drove up to our street and our backyard is on fire, and I was on the phone with my mom and she said, 'Don't you dare go in there!'" college student Christie Adams told ABC News.
Magnussen said officials know the fire started in the brush on the edge of town but they are still trying to determine what sparked it.
Washington State Patrol Chief John Batiste authorized the mobilization of state firefighting resources in battling the blaze, dubbed the Sleepy Hollow Fire.
The Associated Press contributed to this report.