Video obtained by Reuters shows sky-high smoke clouds rising from the burning forest and darkening the sky on Friday (September 2).
The Cal Fire firefighting fleet was seen deploying loads of long-term fire retardants in air tankers and helicopters.
The blaze started on Friday nearly 230 miles (370 km) north of Sacramento.
About 100 homes and buildings were reported destroyed by the fire, authorities said.
California Governor Gavin Newsom proclaimed a state of emergency for Siskiyou County. The emergency declaration will help residents access federal aid and unlock state resources.
Another fire called the Mountain Fire was also simultaneously spreading in Gazelle, located 10 miles (16 km) northwest of Weed.
More than two decades of drought and rising temperatures, exacerbated by climate change, have made California more vulnerable than ever to wildfires. The two most devastating years on record were 2020 and 2021, by the number of acres burned.