CHICO, Calif. — California’s largest wildfire of the year continued to spread Thursday as it blazed through trees in very hot, dry weather.

The Park Fire has burned more than 650,000 acres since it broke out on July 24 near the Sacramento Valley city of Chico and is now burning northward up the western flank of the Sierra Nevada. Containment remains at 34%, according to Cal Fire.

The fire initially spread explosively and quickly became the fourth largest wildfire in California before favorable weather diminished its intensity late last week. The fire flared up again this week due to heat and very low relative humidity.

A large part of the fire area is in the cleanup phase, but isolated fires are still a problem, officials said during the briefing on Thursday morning.

The northeast corner of the fire is the highest priority for firefighting, said fire chief Jed Gaines.

“It’s not time to celebrate,” he said. “We still have several days of hard work ahead of us to maintain what we have.”

The park’s latest fire assessments found 636 structures destroyed and 49 damaged. A local man was arrested after authorities said he started the fire by pushing a burning car into a ravine in a wilderness park outside Chico.

About 100 miles south, a new wildfire in El Dorado County was showing extreme behavior and some Park Fire aircraft were diverted there.

The Crozier Fire, about 10 miles north of Placerville, had burned just over 1 square mile of forest and chaparral by Thursday morning and was only 5% contained.