Firefighters battle an expanding wildfire, June 17, 2016 at the Sherpa Fire near Santa Barbara, California.

More than 1,500 emergency personnel were on the scene of a raging wildfire burning in Southern California Wednesday that forced tens of thousands to flee — a blaze that veteran firefighters said is unlike anything they've seen.

The size of the so-called Blue Cut fire in the Cajon Pass near San Bernardino, around 60 miles east of Los Angeles, was revised late Wednesday from 30,000 acres to just over 25,600 acres, or about 40 square miles, fire officials said. More accurate mapping led to the revision.