The Park Fire outside of Chico in the state's north has been raging since Wednesday in a rural region about three hours' drive northeast of San Francisco.
It has now ravaged more than 370,000 acres (149,700 hectares) according to the Cal Fire agency, making it one of the largest fires in state history.
No casualties have been reported so far, and firefighters benefitted from a slight drop in temperatures over the weekend that allowed them to make some progress, with the fire now 12 percent contained.
Nearly 4,900 firefighters have been mobilized, with 33 helicopters, 400 fire trucks and numerous planes battling the conflagration.
More than 26,000 residents were under evacuation orders Monday afternoon, with authorities calling for extreme caution due to a high risk of fire escalation.
"This fire is extremely unstable and unpredictable," Tehama County Sheriff Dave Kain told a press conference Monday.
"We've seen many places that we thought were going to be safe to move back into erupt in flames again," he added.
The fire progressed during the first 48 hours at the speed of a person walking and has spawned fire tornadoes and generated smoke shaped like mushroom clouds.
'I'm prepared'
The fire was able to spread quickly following multiple heat waves that have struck California and the western United States since the beginning of June.
Vegetation "is still super, super dry," said Daniel Swain, an extreme weather specialist at the University of California, Los Angeles, adding that this was caused by "a month of record-breaking heat and evaporative demand."
While the foothills of the Sierra Nevada Mountains burn regularly, the particular canyons where the Park Fire is located have not seen fires in decades, meaning there is plenty of fuel for the flames.
Despite the massive resources deployed by California, which has special expertise in firefighting, "it's still beyond technology to address a fire at that scope," Swain said.
The enormous Park Fire brings back bad memories: the town of Paradise, where 85 people died in 2018 in the deadliest fire in state history, is only about 12 miles (20 kilometers) from the flames. Its residents have already been put on alert.
Some inhabitants of evacuated towns have chosen to stay until the last minute, such as Justin Freese, who is waiting with a firehose and 10,000 gallons of water at the ready.
"I'm prepared, but I'm not stupid," he told the New York Times. "If there's a 100-foot wall of flames coming, I'm not going to stay put and melt my skin."
Multiple fires
The Park Fire was caused by arson, according to authorities. A 42-year-old man was taken into custody Thursday morning after being spotted pushing a burning car into a ravine, according to the local prosecutor's office.
The United States is presently battling about 100 large fires according to the National Interagency Fire Center, mainly in the west of the country and in particular Oregon, where an airplane pilot fighting the fires died last week.
The smoke generated by the fires has prompted the weather service to issue air quality alerts in many places.
In California, a fire that broke out last week nearly razed the historic gold-mining town of Havilah in the state's center over the weekend, but caused no casualties.
Repeated heat waves and extreme weather events are accelerated by climate change, which is linked to humanity's reliance on fossil fuels, according to scientists.
