A wildfire burning in Plumas County has become California’s largest this season.

The Walker Fire, which started Wednesday, had by 11 a.m. Saturday burned 24,040 acres (37 square miles) and had no containment, CalFire said. Its cause had not been determined.

The burn area is in the Plumas National Forest about 15 miles northeast of Quincy.

The Plumas County sheriff’s office ordered mandatory evacuations for the Genesee Valley Road corridor, the Ward Creek area, the Flournoy Bridge area and from Antelope Lake to the Plumas County line, near Janesville.

Road closures include Janesville Grade, Genesee Valley Road past the Flournoy Bridge, and  Antelope/Indian Creek Road from the Flournoy Bridge to Antelope Dam.

The weather forecast was mixed for fire suppression: cloud cover and lower temperatures were expected to raise humidity, but a predicted increase in southwest winds could increase fire activity, said the incident report Saturday from the Plumas National Forest.