California's largest wildfire of the year continued to spread on Thursday, fueled by extremely hot and dry conditions.

The Park Fire, which ignited on July 24 near Chico in the Sacramento Valley and has since advanced northward along the western edge of the Sierra Nevada, has burned over 660 square miles (1,709 square kilometers). Cal Fire reported that containment remains at 34%.

Initially, the fire's rapid expansion quickly made it the fourth-largest wildfire in California's history. While its intensity was reduced by more favorable weather late last week, it has surged again this week due to the ongoing heat and very low humidity levels.

A significant portion of the Park Fire's burned area was in the mop-up stage, but spot fires continued to pose a challenge, as reported during Thursday morning's operational briefing.

The fire's northeast corner was identified as the top priority for firefighting efforts, according to operations deputy Jed Gaines.

“It’s not time to celebrate,” Gaines stated. “We still have several days of hard work ahead to maintain what we’ve achieved.”

Recent assessments of the Park Fire revealed that 636 structures were destroyed and 49 were damaged. Authorities have arrested a local man, alleging that he started the fire by pushing a burning car into a gully in a wilderness park near Chico.

Approximately 100 miles (160 kilometers) south, a new forest fire in El Dorado County was exhibiting extreme behavior, leading to the diversion of some Park Fire aircraft to the new blaze.

The Crozier Fire, located about 10 miles (16 kilometers) north of Placerville, had burned just over 1 square mile (2.6 square kilometers) of timber and chaparral by Thursday morning and was only 5% contained.