Fire officials say the blaze near Rancho Cucamonga is 53 percent contained and there is no active flame.
No homes have burned in the area east of Los Angeles, and there are no mandatory evacuations.
The blaze began Wednesday morning and quickly surged through dry grass and chaparral.
Winds in the area below the San Bernardino National Forest are around 15 mph, with 25-mph gusts - a far cry from the 70-mph gusts two days ago.
Firefighting aircraft are available to assist crews after being grounded because of fierce gusts.
The National Weather Service predicts the winds will continue to ease and become variable.