A bushfire tore through hillside above Laguna Beach on Thursday as California sizzled under an unseasonable heatwave.

A heatwave was bringing unseasonably high temperatures to California on Thursday, sending sun-worshippers to the beach, but also sparking a brushfire.

Forecasters issued a heat warning for the most populous US state through to Sunday, warning the mercury could hit a height-of-summer 90 Fahrenheit (32 Celsius) in Los Angeles, well above the average for winter.

California, in common with much of the western United States, is enduring a historic drought and wild swings in weather that scientists say is exacerbated by man-made climate change.

"If you warm the planet, you're going to break heat records," said geographer Justin Mankin of Dartmouth College.

Continuing to burn that release planet-heating carbon dioxide is going to make that worse, especially when coupled with natural weather variations.

"Right now you have this kind of prevailing high pressure system that's somewhat amplified," he told AFP.

"You have drier-than-usual conditions at the surface, which just means that more energy will go towards warming up the air rather than evaporating water."

The heat was set to continue into the weekend, when Los Angeles is due to host the Super Bowl, American football's showpiece final.

The previous hottest Super Bowl was in 1973, also in LA, when players trotted out in balmy 84F conditions.

In San Diego, near the Mexican border, temperatures were also expected to hit 90F.

In northern California, the UC Berkeley laboratory in the central Sierra region recorded another record Wednesday: 32 consecutive days without rain, the longest period without precipitation in winter.

A wetter-than-usual December across the state had given hope that the years-long drought might be waning, but 2022 has been dry.

While surfers were happily soaking up California's rays, dozens of people had to flee their homes overnight around Laguna Beach, where a fire tore through 145 acres.

There were no reports of injuries or any property damage in the swanky spot, where million-dollar homes line the roads, but firefighters were urging residents to stay away.

Laguna Beach Mayor Sue Kempf told reporters the flames brought back memories of a 1993 wildfire that destroyed more than 300 homes.

"We no longer have a fire season, we have a fire year," said local fire chief Brian Fennessy.

"It's February 10. This is supposed to be the middle of winter and we're anticipating 80- to 90-degree weather.

"If this is any sign of what's to come throughout the rest of the winter and spring we're in for a long year."