Firefighters brace for mudslides as storm barrels into California burn area

Firefighters battling a growing blaze outside Los Angeles have been bracing for mudslides and flooding as a storm barrels into burn areas.

The remnants of a hurricane that hit Mexico were bringing strong winds on Friday that are likely to fan the Fairview fire, prompting wider evacuation orders.

The10,000-hectare fire, which erupted on Monday at the midpoint of a ferocious heatwave, is continuing to spread, buffeted by "extreme downslope winds" from nearby mountains, fire officials said.

"I have not seen a fire burn like this in Riverside County in my career," said Cal Fire division chief John Crater.

"It's a very stubborn fire. It's doing things that we just haven't seen."

An already widespread evacuation zone was expanded to cover more than 20,000 people as emergency managers tried to out-flank the fire and get ahead of the winds.

"Winds will increase from the east... Ember cast will dramatically increase as the strong 65+ kph winds enter the area," Cal Fire warned.

"Long-range spotting over a mile will be possible with Probability of Ignition at 85 percent."

"Ember cast" and "spotting" refer to burning materials jumping from the main fire and settling elsewhere, igniting and spreading the blaze.

Sheriff's deputies were going door to door to urge residents to get out of harm's way.

At least two people have already died in the blaze, apparently trapped by fast-moving flames as they tried to flee.

READ MORE: California firefighters struggle as wildfire doubles in size

'We have challenging days ahead'

Meanwhile, Tropical Storm Kay, which made landfall in Mexico as a hurricane on Thursday was rolling north and expected to bring heavy rain to parts of California and Arizona.

Forecasters at the National Weather Service said up to 18 centimetres of rain could fall in the area around the Fairview fire, creating the risk of flash flooding and mudflows in areas where burned-out soil cannot absorb the sudden downpour.

"We could go from a fire suppression event into significant rain, water rescues, mudslides, debris (flows)," Jeff Veik of Cal Fire's Riverside Unit told a community meeting.

"We have challenging days ahead."

The storm, which by Friday afternoon was lashing southern California with gusts over 160 km per hour, looked set gradually to bring an end to the punishing heatwave that has enveloped a large chunk of the western United States for more than a week.

But temperatures –– some of which have exceeded 43 Celsius in places for multiple days –– remained high in central and northern California.

"Overnight lows will continue to rival records this weekend as the increased cloud cover traps warm air at the surface," the National Weather Service said.

"Approximately 29 million Americans are currently under an Excessive Heat Warning."

The warm and dry air was also increasing the chances of fires over the greater West, a risk that was being further elevated by winds that are whipped up as pressure systems move around.

Climatologists predict that as the Earth continues to heat because of the unceasing burning of fossil fuels, these conditions will further worsen.

READ MORE: California blaze destroys 100 homes as thousands flee