Forest Service spokeswoman Vicki Collins says the blaze grew by 500 acres overnight and could become even more active Monday afternoon when highs could reach 98 degrees with 20-mph wind gusts.
The fire is burning in a remote area of the San Rafael Wilderness, about 25 miles east of Santa Maria.
No homes are threatened and no injuries are reported but a dozen campgrounds were evacuated after the fire erupted on Saturday.
Collins says about 600 firefighters aided by water-dropping aircraft are fighting the blaze in steep terrain. The fire is burning through 6-foot-tall chaparral that last burned in 1922.
The cause hasn't been determined.