The flames ripped through northwest Shasta County then spread southeast, sweeping across the Sacramento River overnight and roaring toward the city limits of Redding, which is home to 92,000.
The so-called Carr Fire is "taking down everything in its path", said Scott McLean, a CalFire spokesman for the crews battling the blaze.
At least 15 structures have been destroyed in the Carr Fire. "It was like a Tasmanian devil", McLean told Reuters.
But he says extreme winds are expected again Friday afternoon and says people in the area should be ready to evacuate.
Almost 5,000 homes were listed by CalFire as threatened.
Weather conditions are exacerbating the fire, with hot and windy weather fanning the flames, causing the wall of fire to grow larger.
Winds were forecast to reach speeds of 40 km per hour, with temperatures reaching 43 C, conditions that could fan the flames further and hamper efforts to combat the blaze.
Hospital staff is preparing to evacuate the facility if needed, a Cal Fire spokesman said. "Stress, as far as heat exhaustion and heat stroke, are really serious situations that a firefighter can get into, so the tactics kind of get hindered by temperatures that high".
(Picture: AP) A house burns during the Carr fire in Redding, California (Picture: AFP) More than 1,700 firefighters are battling the blaze, Cal Fire said in an evening advisory.
The Carr Fire is one of 88 burning nationally, mostly across the American west.
A huge forest fire continued to grow outside Yosemite National Park.
Wildfires have blackened an estimated 4.15 million acres (1.68 million hectares) in the United States this year.
The fire was 31 square miles (80 square kilometers) Thursday morning.
Never Mind. Fire has crossed the Sacramento River.
Meanwhile, in southern California, the Cranston fire has consumed 11,500 acres and is also only 3% contained. Fire officials said about 600 homes are in immediate danger as thousands of people are being evacuated.
Firefighters tried in vain to build containment around the blaze Thursday, but flames kept jumping their lines, he said. "We are just waiting it out. crossing our fingers and hoping for the best".
Roads out of the city, which is home to 90,000 people, have been left jammed with motorists trying to escape the flames. Power went out at the Record Searchlight newspaper, while the news station KRCR had to evacuate its offices for a period of time.
More than 3,000 customers had lost power in the area, according to utility PG&E. The fire was just 25 percent contained.
Brandon N. McGlover, 32, of Temecula was detained Wednesday and is facing five counts of arson to wildland, the Riverside County Fire Department and Cal Fire said.