The quick-moving blaze had burned up to 20 acres as of 9:30 p.m. near the intersection of Shinn and Mount Baldy roads, according to Deputy Fire Chief Tim Dunfee of the Angeles National Forest agency.
The fire was five percent contained and had burned 12 acres as of late Friday night.
"Tonight we noticed a battle between topography and winds," Dunfee said. "Winds were downslope about five miles per hour, but the fire was wanting to burn upslope, so the two were battling each other, moderating fire behavior."
No injuries were reported and no structures were immediately threatened, fire officials said.
A water-dropping helicopter from the Los Angeles County Fire Department's Air Operations Section was assisting at the scene.
MT BALDY: #SBCoFD on scene VEGETATION FIRE Shinn Rd / Mt Baldy Rd. 4-5 acres on the hillside. Heavy fuel with rapid rate of spread. Threat to National Forest (poss already on the forest). @LACOFD @Angeles_NF also responding. No Immed structure threat. BC119 is MtBaldy IC. ^eas— SB County Fire (@SBCOUNTYFIRE) November 3, 2018
The closest community, Claremont, was four to five miles away from the fire. No evacuations were ordered.
Mt. Baldy Road, Mountain Avenue and Euclid streets were closed, with people asked to use Glendora Ridge Road as an alternate.