LOS ANGELES — Ash fell like snow and heavy smoke had residents gasping for air Monday as a huge Southern California wildfire exploded in size, becoming the fifth largest in state history and driving celebrities from a wealthy hillside enclave.

Tens of thousands have fled their homes as flames churn through foothill towns near Santa Barbara, the latest flare-up after a week of wind-fanned wildfires throughout the region.

With acrid smoke thick in the air, even residents not under evacuation orders took the chance to leave, fearing another shutdown of a key coastal highway that was closed intermittently last week.