Nearly 1,900 firefighters were battling the Ponderosa Fire in rugged, densely forested terrain as it threatened 3,500 homes in the towns of Manton, Shingletown and Viola, about 170 miles north of Sacramento.
"These are the largest number of homes we've had threatened so far this year," state fire spokesman Daniel Berlant said. "The grass, brush and timber up here are so dry, and once the lightning with no rain struck, the flames began to spread quickly."
The fire has destroyed seven homes while blackening more than 25 square miles. It was just 30 percent contained after beginning Saturday.
The fire forced the closure of Highway 44 and other roads, and prompted the declaration of an emergency in Shasta County. The Red Cross set up an evacuation center at a sports complex in Redding, where dozens of people, from the elderly to infants, as well as about a dozen dogs, were given shelter.
One evacuee, Bonnie Maloy, who escaped her home in Shingleton, along with her husband Bill, described the scene as they fled the flames.
"Frantic at first, then I said, 'Let's calm down,' and we got everything that's important, things we couldn't replace: animals, kids, photo albums," she said.
Another massive wildfire burning to the south in Plumas National Forest since July 29 grew larger over the weekend as strong winds pushed the flames past fire lines established late last week.
The blaze, about 120 miles north of Sacramento, has consumed more than 79 square miles and was threatening about 900 homes. It was 37 percent contained.
Elsewhere in California, a wildfire in Lassen Volcanic National Park was 65 percent contained after consuming more than 45 square miles. Officials expected firefighters would have the blaze contained by Tuesday.
In Mendocino County, a wildfire that started Saturday had consumed about 9 square miles. That blaze was sparked by lightning and was burning in a remote area of thick timber and rugged terrain, making it difficult for fire crews to access.
In Idaho, firefighters dug in to defend the town of Featherville against the raging Trinity Ridge Fire that has chewed through 140 square miles in the central region of the state. Flames had yet to reach Featherville, which was evacuated two days ago. But if they do, firefighters said they were ready with a planned burnout operation intended to slow the blaze.
Thunderstorms were expected and could cause gusty, shifting winds around the fire.
A little to the north, Idaho's largest wildfire, the Halstead Fire, which was ignited by lightning in late July, had burned 143 square miles.
In Washington state, firefighters hoped to fully contain a wildfire that burned dozens of homes in Cle Elum, about 75 miles east of Seattle. Crews across the state expected thunderstorms that could bring moisture along with the possibility of lightning and additional fires.
The fire broke out a week ago at a bridge construction project and has burned nearly 36 square miles of grass, sagebrush and timber in rural areas. Most of Eastern Washington was under a fire watch Monday due to high temperatures and low humidity.
Smaller fires were burning in Colorado and Utah after being sparked by lightning over the weekend.
Collins reported from San Francisco. Associated Press writers John S. Marshall in San Francisco, Lynn DeBruin in Salt Lake City, Shannon Dininny in Yakima, Wash., and Jessie Bonner in Boise, Idaho, contributed to this report.
Western wildfires update
Washington state • Firefighters hoped to fully contain a wildfire that has burned dozens of homes in Cle Elum, about 75 miles east of Seattle. The fire broke out a week ago at a bridge construction project and has burned nearly 36 square miles of grass, sagebrush and timber in rural areas.
Idaho • Fire managers planned to meet with residents in Pine after an evacuation of Featherville was ordered over the weekend due to smoke from an approaching wildfire. About 1,100 firefighters battled the Trinity Ridge Fire, which has burned 140 square miles and was threatening homes.