LOS ANGELES — Flames raced down a steep hillside “like a freight train,” leaving smoldering remains of homes and forcing thousands to flee the wildfire churning through tinder-dry canyons in Southern California, authorities said Sunday.

The fire that has destroyed at least 18 homes in northern Los Angeles County gained ferocious new power two days after it broke out, sending so much smoke in the air that planes making drops on it had to be grounded for part of the afternoon.

“For this time of year, it’s the most extreme fire behavior I’ve seen in my 32-year career,” County fire Chief Daryl Osby said.

About 300 miles up the coast, crews were battling another fire spanning more than 17 square miles that destroyed six homes on Sunday and forced evacuations outside the scenic Big Sur region.

The Southern California blaze has blackened at least 46 square miles of brush on ridgelines near the city of Santa Clarita. Osby said the size estimate could grow considerably once better assessment is done.

Planes were unable to make drops over the fire for a long stretch of the afternoon before resuming for a few hours before dusk. Helicopters released retardant around the perimeter of the fire all day and would continue into the night.

“The fire’s just doing what it wants right now,” U.S. Forest Service spokesman Nathan Judy said. “We have to stick back, let it do what it wants to and attack it where we can.”

Juliet Kinikin said Sunday there was panic as the sky became dark with smoke and flames moved closer to her home a day earlier in the Sand Canyon area of Los Angeles County.

“And then we just focused on what really mattered in the house,” she told The Associated Press.

Kinikin grabbed important documents and fled with her husband, two children, two dogs and three birds. They were back at home Sunday, “breathing a big sigh of relief,” she said.

Residents of thousands of homes were evacuated as shifting winds were pushing flames northeast through Angeles National Forest, authorities said.

Lois Wash, 87, said she and her daughter and her dog evacuated, but her husband refused.

“My husband’s stubborn as a mule, and he wouldn’t leave,” Wash told KABC-TV.