Wildfire burning north of Los Angeles balloons overnight

SANTA CLARITA, Calif. -- A wildfire burning in wilderness north of Los Angeles expanded significantly overnight to more than 8½ square miles.

Much of the sky over greater Los Angeles is filled with heavy smoke Saturday morning and ash has fallen in some areas.

Authorities have issued an advisory warning that air quality may reach unhealthy levels in central Los Angeles and adjacent valleys.

The fire erupted Friday afternoon in the Sand Canyon area near State Route 14 as the region was gripped by triple-digit heat and very low humidity levels. Some 300 homes have been ordered evacuated.

The blaze is being battled by 300 firefighters from Los Angeles County and the Angeles National Forest as well as a fleet of helicopters and airplanes.