Winds in the “fire whirl” created on July 26 near Redding reached speeds of 143 mph (230 kph).
The speed rivalled some of the most destructive Midwest tornados, National Weather Service meteorologist Duane Dykema said.
The whirl measured a three on the five-level Enhanced Fujita scale, which scientists use to classify the strength of tornados, he said.
California has not recorded a tornado of that strength since 1978.
That fire continues to burn about 100 miles (160 kilometres) south of the Oregon border as firefighters there and throughout northern California brace for worsening conditions this weekend.
The 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.
“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 include the massive blaze near Redding and two fires burning next to each other around Clearlake about 100 miles (160 kilometres) north of San Francisco.
The Redding fire has grown to 206 square miles and has destroyed 1,060 homes and many other structures.
Two firefighters and four other people have been killed since the blaze, which ignited on July 23, raced with extraordinary fury toward the region’s largest city.
Wildfires typically create whirls but rarely of the strength of the one recorded on July 26, Mr Dykema said.
Whirls are created when hot air rises and twists tightly, he said.
The hotter the fire, the faster the air rises and the tighter it twists until it takes off as a tornado.