GREENVILLE, Calif. — A wind-driven wildfire tore through a Northern California mountain town, leaving much of the downtown in ashes as crews braced for another explosive run of flames in the midst of dangerous weather.

The Dixie Fire, swollen by bone-dry vegetation and 40-mph gusts, raged through the northern Sierra Nevada town of Greenville Wednesday evening. A gas station, hotel and bar were among many structures gutted in the town, which dates to California's Gold Rush era and has some buildings more than a century old.

"We did everything we could," fire spokesman Mitch Matlow said. "Sometimes it's just not enough."

As the fire's north and eastern sides exploded, the Plumas County Sheriff's Office issued a Facebook posting warning the town's approximately 800 residents: "You are in imminent danger and you MUST leave now!"

The three-week-old blaze was the state's largest wildfire and had blackened well over 435 square miles (1,126 square kilometers), burning dozens of homes before making its new run.

Early in the week, some 5,000 firefighters had made progress on the blaze, saving some threatened homes, bulldozing pockets of unburned vegetation and managing to surround a third of the perimeter.

More fire engines and bulldozers were being ordered to bolster the fight, Matlow said. On Wednesday, the fire grew by thousands of acres and an additional 4,000 people were ordered to evacuate, bringing nearly 26,500 people in several counties under evacuation orders, he said.

Red flag weather conditions of high heat, low humidity and gusty afternoon and evening winds erupted Wednesday and were expected to be a continued threat through Thursday evening.

The trees, grass and brush were so dry that "if an ember lands, you're virtually guaranteed to start a new fire," Matlow said.

The blaze was running parallel to a canyon area that served as a chimney, making it so hot that it created enormous pyrocumulus columns of smoke. These clouds bring chaotic winds, making a fire "critically erratic" so it's hard to predict the direction of growth, he added.

Dawn Garofalo fled with a dog and two horses from a friend's property near Greenville, and watched the soaring cloud grow from the west side of Lake Almanor.

"There's only one way in and one way out," she said. "I didn't want to be stuck up there if the fire came through."

From her campsite on the dry lake bed, she watched the fire glowing on the horizon before dawn.

About 150 miles (240 kilometers) to the west, the lightning-sparked McFarland Fire threatened remote homes along the Trinity River in the Shasta-Trinity National Forest. The fire was only 5% contained after burning through nearly 25 square miles (65 square kilometers) of drought-stricken vegetation. 

Similar risky weather was expected across Southern California, where heat advisories and warnings were issued for interior valleys, mountains and deserts for much of the week. 

Heat waves and historic drought tied to climate change have made wildfires harder to fight in America's West. Scientists say climate change has made the region much warmer and drier in the past 30 years and will continue to make weather more extreme and wildfires more frequent and destructive.

More than 20,000 firefighters and support personnel were battling 97 large, active wildfires covering 2,919 square miles (7,560 square kilometers) in 13 U.S. states, the National Interagency Fire Center said.