State officials on Sunday were urging residents to limit their power usage for the fifth day in a row as energy demand spiked and temperatures were still on the rise.

The worst of the heat was concentrated in the Central Valley on Sunday, where temperatures could climb to 109 degrees Fahrenheit. The thermometer could hit 115 degrees by midweek, the National Weather Service said, warning residents to stay indoors to avoid heat-related illness.

“The heatwave begins in earnest today with dangerous temperatures now forecast to extend through the end of the week,” National Weather Service Sacramento wrote on Twitter on Sunday.

The southern California city of San Diego, which set a record temperature on Saturday of 95 degrees, could set another record on Sunday, said National Weather Service forecaster Tony Fracasso, although a chance of afternoon thunderstorms could offer some relief.

The California Independent System Operator (ISO), which oversees the state’s electric grid, extended a “flex alert” to a fifth day, asking state residents to set their thermostats to 78 degrees or higher, avoid using major appliances and turn off lights to conserve energy.

“Sunday, Monday and Tuesday, in particular, are shaping up to be the most difficult of this heatwave,” the agency said in a news release.

It added that the state’s ongoing wildfires and potential new blazes could further strain the power grid by crippling lines and generators. More than two decades of drought and rising temperatures, exacerbated by climate change, have made California more vulnerable than ever to wildfires.

In northern California’s Siskiyou County, where firefighters were battling the fast-moving Mill Fire that prompted thousands to evacuate their homes, the high temperature forecast for Sunday was 95 degrees. Temperatures were expected to top 100 degrees in the coming days. The fire had burnt more than 4,000 acres and was 25% contained as of Sunday morning, according to the California Department of Forestry and Fire Protection.

(Reporting by Gabriella Borter; Editing by Mark Porter.)