GREENVILLE, Calif. (AP) — California’s largest wildfire continued to grow Wednesday as 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. 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 some 15,000 people, 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 across Plumas and Butte counties.

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

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 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 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 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 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, was 84% contained and 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.

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