California’s largest wildfire continued to grow Wednesday while thousands of firefighters prepared for a tougher fight as dangerous weather returns.

A red flag warning was issued through Thursday because of hot, bone-dry conditions with winds up to 40 mph (64 kph). That could drive flames through timber, brush and grass, especially along the northern and northeastern sides of the vast Dixie Fire.

“I think we definitely have a few hard days ahead of us,” said Shannon Prather with the U.S. Forest Service.

Firefighters were able to save homes and hold large stretches of the blaze. But flames jumped perimeter lines in a few spots Tuesday, prompting additional evacuation orders for about 15,000 people east of Lake Almanor, fire officials said.

Firefighters were protecting the small Northern California mountain community of Greenville as the 3-week-old fire grew to over 428 square miles (1,108 square kilometers) across Plumas and Butte counties.

Heat from the flames created a pyrocumulus cloud, a massive column of smoke that rose 30,000 feet (10,000 yards) in the air, said Mike Wink, a state fire operations section chief.

Dawn Garofalo watched the cloud grow from the west side of the lake, where she fled with a dog and two horses, from a friend’s property near Greenville.

“There’s only one way in and one way out. I didn’t want to be stuck up there if the fire came through,” Garofalo said.

From her campsite on the lake bed, she watched the fire glowing on the horizon before dawn. “The flames were huge. They must have been 500 feet high. Scary,” she said.

The fire has threatened thousands of homes and destroyed 67 houses and other buildings since breaking out July 14. It was 35% contained.

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. It had burned fiercely 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 the American 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.

Montana on Tuesday had 25 active large blazes, followed by Idaho with 21 and Oregon with 13. California had 11.

In Hawaii, firefighters gained control over the 62-square-mile (160-square-kilometer) Mana Road Fire that forced thousands of people to evacuate over the weekend and destroyed at least two homes on the Big Island.

Oregon’s Bootleg Fire, the nation’s largest at 647 square miles (1,676 square kilometers), was 84% contained. Firefighters were busy mopping up hot spots and strengthening fire lines.

“Crews are working tirelessly to ensure we are as prepared as we can be for the extreme fire weather forecast for the next couple days,” a U.S. Forest Service update said.

Correction: The blaze in Hawaii is called the Mana Road Fire, not the Nation Fire. This story had been updated.