LOS ANGELES — The largest wildfire in California's recorded history expanded to almost 300,000 acres on Tuesday, postponing the start of the school year as ash choked the air and thousands of people remained driven from their homes.

The Mendocino Complex, encompassing two fires lapping the northern and western shores of Clear Lake about 80 miles northwest of Sacramento, added 17,000 acres overnight to reach 290,000 acres, authorities said.

The Mendocino Complex fire has nearly doubled in size since Saturday. It supplanted last year's Thomas Fire on Monday as the biggest on record in California. If it were a city, it would be the 14th largest by area in the United States.

#MendocinoComplex wildfire now the largest in California history. This image shows the fire perimeter overlaid across the SF bay area for scale. pic.twitter.com/paA5zLohpc — NWS Bay Area (@NWSBayArea) August 7, 2018

Containment was reported at 34 percent on Tuesday. The California Department of Forestry and Fire Protection, or Cal Fire, which had initially projected that it would be contained by Aug. 15, pushed the date back to Sept. 1 on Monday, blaming high temperatures, limited access and heavy, thick, dry timber and ground fuel.

With 13,000 residents under evacuation orders in the region, some school districts delayed the start of the school year as smoke hovered and ash continued to fall.

"We really can't do a lot until we're sure the ash and smoke are going to leave," said April Lieferman, superintendent of the Lakeport Unified School Board in Lake County, which pushed this week's opening of classes back by at least two weeks.

"Our district office hasn't been up and running to start taking inventory," Lieferman told NBC affiliate KCRA of Sacramento.

Sabrina Bailey, who lives in Lake County, told the station that she was walking out of her bank "and I was just like, oh, it's started snowing again — everybody's car, it's everywhere." The "snow" was ash.

"We're getting used to it, and that's not a good thing," Bailey said.

The Mendocino Complex is just one of 17 wildfires across California that the state Office of Emergency Services classifies as major, from the Natchez Fire straddling the California-Oregon border to the arson-caused Cranston Fire in San Bernardino National Forest in the south.

Almost 14,000 firefighters are on duty battling the fires, which have burned 624,000 acres, the emergency services office said Tuesday. That's an area 30 percent larger than Jacksonville, Florida, the fifth-biggest city in the United States.

"I can remember a couple of years ago when we saw 10 [thousand] to 12,000 firefighters in the states of California, Oregon and Washington and never the 14,000 we see now," Scott McLean, a spokesman for Cal Fire, told NBC Bay Area.

"I loved it, loved it," Emily Scheidemann said of her family's home in Upper lake, which was devoured by the River Fire, part of the Mendocino Complex.

"I've just been kind of sitting there and staring off into space, just like, 'This is not real,'" Scheidemann told NBC Bay Area as she described watching her home collapse. "I just want to go home."