HEMET, United States - California firefighters were able to beat back a massive wildfire outside Los Angeles after a tropical storm brought rains and cooler temperatures, US authorities said on Saturday.

The Fairview Fire was 40 per cent contained as of Saturday evening after forcing evacuation orders and leaving two people dead, fire officials said.

The blaze erupted on Monday at the midpoint of a ferocious heat wave in the southwestern United States, scorching 28,000 acres and destroying more than 20 buildings.

The remnants of storm Kay, which made landfall on Thursday in Mexico as a hurricane before rolling north up the Pacific Coast, brought rains that helped calm the fire.

"Fire activity has been greatly reduced due to the moisture from Tropical Storm Kay," a statement from Cal Fire, the California Department of Forestry and Fire Protection, said.

Authorities warned, however, that the rains brought a 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)," Mr Jeff Veik of Cal Fire's Riverside Unit said on Friday.

The western United States is more than two decades into a historic drought that scientists say is being worsened by human-made climate change.

Much of the countryside is parched and overgrown, creating the conditions for hot, fast and destructive wildfires. AFP