Click in the Eyewitness News Story Window above to watch the accompanying video to this story.
The fire was spotted just after 4:30 a.m. Tuesday in the Bear Creek Canyon area of Mount Baldy.
Some 450 firefighters were on scene Tuesday night. During the day, they were aided by water-dropping helicopters, but air units have been grounded for the night. Crews have been fighting the fire mostly from the air because it's burning in a very steep area.
The big concern is the wind. Firefighters say they're dealing with gusts ranging from 40 to 50 miles an hour.
The fire is threatening about 40 homes in Bear Canyon, most of them vacation cabins. Fire crews are protecting the structures, but all of them have been evacuated as a precaution.
A father and daughter hiking in the area Tuesday morning had to be airlifted to safety after getting trapped. They are reportedly uninjured.
One authority told Eyewitness News that there hasn't been a fire in Mount Baldy since 1975, so there's a lot of dry terrain and fuel.
"The concern right now is as the day heats up, the sun will dry out the fuels and with the wind, the fuels will burn faster and hotter as the day heats up and the winds progress. Right now we currently have 15 to 20 mile an hour steady winds with gusts up to 30-40 sometimes even 50 mile an hour winds at the ridgeline," said L.A. County Fire Dept. Inspector Frank Garrido.
Since the fire in burning in a very steep area, hand crews have had to hike up the mountainside to fight it from the ground. The fire is being mostly fought from the air.
There's no word on what started the blaze.
Eyewitness News Reporters Subha Ravindhran and Wendy Burch contributed to this report.