Twenty evacuees were taken to area hospitals, the Madera County Sheriff said on Twitter on Sunday, as the Creek Fire that started on Friday night rapidly grew to burn some 36,000 acres, forcing evacuations and road closures in the Fresno area in central California.

As of Sunday morning, nearly 15,000 firefighters were battling 23 fires across the state, according to the California Department of Forestry and Fire Protection (CalFire).

Three major fires, including the Creek Fire, were burning in Fresno, San Bernardino and San Diego counties, the agency said in a statement, adding it had increased staffing in preparation for “critical fire weather.”

A dangerous heat wave was baking swathes of the western United States through the weekend, and many locations in California registered record-high temperatures on Saturday.

The National Weather Service warned that “sweltering” heat would continue on Sunday, creating ideal conditions for wildfires.

State officials on Sunday repeated calls to Californians to turn off appliances and lights to help avoid blackouts from an overwhelmed power grid.

Southern California Edison Co (SCE), which services 5 million customer accounts in the region, was advising customers of potential rotating outages between 4 p.m. and 9 p.m. on Sunday.

Rotating outages — in which utilities purposely cut power temporarily to avoid broader outages — are rare. They last occurred in the region in August but before that had not occurred in 20 years, said SCE spokesmen Reggie Kumar.

(Reporting by Maria Caspani in New York, additional reporting by Nicholas Brown in New York, Editing by Lisa Shumaker)