The National Weather Service issued warnings for critical fire weather conditions into Saturday, saying a series of dry low-pressure systems passing through the region would bring afternoon wind gusts.
Much of the destruction was caused by a tornado of flames that the National Weather Service said reached 143 mph (230 kph) on July 26.
NWS meteorologist Duane Dykema said the “fire whirl” had an intensity rivaled by some of the most destructive Midwest tornadoes recorded.
Dykema said the whirl uprooted trees and tore roofs from home.
Dykema said the whirl measured a 3 on the five-level Enhanced Fujita scale, which scientists use to classify the strength of tornados. California has not recorded a tornado of that strength since 1978.
“This is a particularly dangerous situation with extremely low humidity and high winds. New fires will grow rapidly out of control, in some cases people may not be able to evacuate safely in time should a fire approach,” the weather service said in its bulletin for the Mendocino area north of San Francisco.
Forecasters said areas with the highest threat included a massive blaze near the city of Redding and the Mendocino Complex of two fires north of San Francisco.
The 206-square-mile (533-square-kilometer) Carr Fire 100 miles (160 kilometers) south of the Oregon border was 39 percent contained after destroying 1,060 homes and many other structures.
Two firefighters and four other people have been killed since the blaze, which ignited July 23, raced with extraordinary fury toward the region’s largest city. More than 1,300 homes remained threatened.
Dykema said wildfires typically create whirls, but rarely of the strength of the one recorded July 26.
He said whirls are created when hot air rises and twists tightly. The hotter the fire, the faster the air rises and the tighter it twists until it takes off as a tornado, he said.
To the southwest of Redding, new evacuations were ordered late Thursday at the Mendocino Complex, where twin fires have ravaged a combined 240 square miles (621 square kilometers), destroyed 41 residences and threatened 9,200 homes.
One of the fires rapidly expanded overnight.
In Sierra Nevada, firefighters achieved 41 percent containment of a 115-square-mile (298-square-kilometer) forest fire that has shut down Yosemite Valley and other adjacent portions of Yosemite National Park at what is normally the height of summer tourism.
Two firefighters have been killed there since a fire erupted just west of Yosemite in the Sierra National Forest.