VENTURA, Calif. >> Raked by ferocious Santa Ana winds, explosive wildfires northwest of Los Angeles and in the city’s foothills burned a psychiatric hospital and scores of other structures today and forced the evacuation of tens of thousands of people.

One of the blazes broke out Monday in Ventura County and grew wildly to more than 70 square miles in a matter of hours, county Fire Chief Mark Lorenzen said. It was fanned by winds clocked at well over 60 mph that grounded firefighting helicopters and planes.

A smaller fire erupted on the northern edge of Los Angeles, threatening the Sylmar and Lakeview Terrace neighborhoods, where residents scrambled to get out as heavy smoke billowed over the city, creating a health hazard for millions of people.

Just weeks ago, wildfires that broke out in Northern California and its famous wine country killed 44 people and destroyed 8,900 homes and other buildings.

At least 150 structures burned in the Ventura County fire, officials said. TV reports showed mansions and modest homes in flames, along with Vista del Mar Hospital, which treats patients with mental problems or substance abuse, including veterans with post-traumatic stress syndrome.

More than 27,000 people were evacuated and one firefighter suffered bumps and bruises in a vehicle accident in Ventura County. Authorities initially reported one death, but then retracted that, saying a dead dog but no person was found in an overturned car.

The fire erupted near Santa Paula, a city of some 30,000 people about 60 miles northwest of Los Angeles. Many of the evacuated homes were in that city.

“We had the fire come through here, pretty dramatically, all night long,” said Karen Heath-Karayan, who stayed up with her husband to douse flaming embers that rained on their home and small lot where they sell Christmas trees. “It was really scary.”

They were ordered to evacuate as flames got within about 100 yards, but they decided to stand their ground to protect their property, where they have chickens and goats.

They hosed down their roof and hit hot spots before winds pushed the fire over a hill toward neighboring Ventura, a city of 106,000 where more people were ordered to clear out.

“It was just exponential, huge growth because the winds, 50 miles an hour out of the east, were just pushing it and growing it very, very large, very quickly,” Lorenzen said shortly after sunrise.

He said daylight would allow air tankers and helicopters to go into action.

Thomas Aquinas College, with about 350 students, was evacuated.

The smaller fire on the northern edge of Los Angeles was estimated at more than 6 square miles (15 square kilometers) and had burned homes, though no damage estimates were released. About 2,500 homes were ordered evacuated.

The flames were driven by Southern California’s dry and gusty Santa Ana winds, which have contributed to some of the region’s most disastrous wildfires. They blow westward, from inland areas toward the coast, speeding up as they squeeze through mountain passes and canyons.

Nearly 180,000 customers in the Ventura County lost power, and schools in the district were closed. Some firefighting efforts were hampered when pumping stations lost power.

Brian Melley in Los Angeles contributed to this report.