Both state and local energy officials are asking residents to conserve electricity amid what is expected to be the state's most significant heat wave so far this year.

The California Independent System Operator, which manages the state's power grid, issued a "flex alert" calling for voluntary conservation from 4 p.m. to 9 p.m. both Wednesday and Thursday, with high temperatures expected to push up demand and tighten power supplies.

The operator, widely known as California ISO, said additional flex alerts are likely as record-setting temperatures are forecast across much of the West, including areas less accustomed to triple-digit heat.

The state announcement comes two days after the Imperial Irrigation District issued what it was calling a "Conserve Alert" calling on customers to voluntarily conserve energy during the same hours through Friday. 

The IID provides power to Thousand Palms, Bermuda Dunes, La Quinta, Indio and Coachella in the Coachella Valley, as well as all of Imperial County and parts of eastern San Diego County. Highs are also expected to surpass 110 degrees through Labor Day in the Imperial Valley.

The valley's other major electricity supplier, Southern California Edison, had not issued any similar requests, but also called for conservation after the state flex alert. Edison provides power to a massive swath of Southern California, including the western and central Coachella Valley from Palm Springs to Palm Desert.

Excessive-heat warnings expanded to all of Southern California and northward into the Central Valley on Wednesday, and were predicted to spread into Northern California later in the week. The warnings now extend as far north as Redding and cover many areas that are not accustomed to seeing triple-digit temperatures, as the Coachella Valley is. 

In what’s likely to be the most extensive heat wave so far in the West this year, temperatures in Northern California are expected to be 10 to 20 degrees warmer than normal through Tuesday. In Southern California, temperatures are expected to be 10-18 degrees warmer than normal.

Death Valley is currently forecast to have a high of 126 degrees on Saturday, which would tie the highest temperature ever recorded on Earth in the month of September.

The flex alert time, 4 p.m. to 9 p.m., is when the grid is most stressed because demand is higher and solar energy production is declining or unavailable.

Suggested steps for conserving include setting thermostats no lower than 78 degrees when home, using floor or ceiling fans but turning them off when leaving a room, using shades and blinds to block the sun and turning off unnecessary lights.

California ISO said in a statement that it was taking measures to bring all available energy resources online, including issuing an order restricting maintenance from noon to 10 p.m. daily through Tuesday, Sept. 6.

The peak load for electricity demand in California is projected to exceed 48,000 megawatts on Monday, the highest of the year, the grid operator said.

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The heat wave arrived amid concern about California’s power grid. In August 2020, a record heat wave caused a surge in power use for air conditioning that overtaxed the grid. That caused two consecutive nights of rolling blackouts, affecting hundreds of thousands of residential and business customers.

Gov. Gavin Newsom has proposed extending the life of the state’s last operating nuclear power plant by five years to maintain reliable power supplies in the climate change era. The proposal would keep Pacific Gas & Electric’s Diablo Canyon Nuclear Power Plant, which is on the Central Coast near San Luis Obispo, running beyond a scheduled closing by 2025.

Forecasters, meanwhile, warned of triple-digit temperatures with little overnight relief, as well as elevated risk of wildfires in much of the West.

“The big weather story this week will be a prolonged and possibly record heat wave building across much of the Western U.S. associated with a strong upper level ridge,” the National Weather Service wrote.

In Palm Springs, the heat was predicted to peak on Wednesday with a forecasted high of 116 degrees. Highs are expected to be 111 or higher every day through Tuesday.

The Associated Press contributed to this report.

Paul Albani-Burgio covers breaking news and the City of Palm Springs. Follow him on Twitter at @albaniburgiop and via email at paul.albani-burgio@desertsun.com.