A giant pyrocumulus cloud has formed above the Bridge Fire, which is burning in the San Gabriel mountains behind downtown Los Angeles.

Wildfires that have burned dozens of mountain homes near Los Angeles slowed on Thursday with cooler temperatures but authorities warned the blazes still threatened suburbs.

Driven by triple-digit heat, gusting winds and tinder-dry vegetation, the three fires burned at speeds firefighters have never witnessed, scorching over 110,000 acres (44,510 hectares) - an area twice the size of Seattle.

The Bridge Fire, California's largest current wildfire, swept through communities in the San Gabriel Mountains less than 40 miles (65 km) northeast of central Los Angeles, where people priced out of the city have built homes.

The 51,000-acre (20,640-hectare) blaze destroyed over 40 houses and cabins in Mount Baldy and Wrightwood as well as damaging lifts at the Mountain High ski area, authorities said.

Fire growth eased as the high, for Los Angeles fell to 81F (27C), a cooling trend expected to last through the weekend.

"We're really expecting to make some great progress on this fire today due to the coastal marine influence, the lowering of temperatures," said Lisa Cox, a spokesperson for the response team, referring to a process that raises moisture levels.