The fire in the Sonoma County wine region north of San Francisco grew to more than 15 square miles before dawn and authorities ordered evacuations near the small community of Geyserville.

There was no immediate information about what caused the fire, but wildfire risk was extremely high as humidity levels plunged and gusty winds up to 70 mph hit the region.

The Pacific Gas & Electric Co. utility on Wednesday began rolling blackouts stretching from the Sierra foothills in the northeast to portions of the San Francisco Bay Area in a bid to keep the electrical grid from causing fires due to wind that can send power lines toppling, starting fires.

The blackouts impacted a half-million people — or nearly 180,000 customers — in 15 counties, and PG&E warned that a second round of outages could occur over the weekend when winds return to the region.