The California Independent System Operator, which manages most of the state's grid, said a so-called Flex Alert will be in effect from Monday through Tuesday evening. The National Weather Service has issued an excessive heat warning in the region for those days.
Record-breaking highs for the date were recorded in several areas Sunday. Lancaster's 113 degrees broke a 1950 record of 105. Borrego Springs' 120 degrees broke a 1973 record of 118 and Paso Robles' 110 degrees broke a 1996 record of 107. Redding reported a high of 110, Sacramento had 107 while Fresno saw 109.
The weather service's thermometer recorded a peak temperature of 128 degrees in Death Valley National Park, which ties the record for the hottest June day anywhere in the country. However, the Los Angeles Times reports that the National Park Service thermometer—200 yards away—recorded a temperature of 129.9, which shatters the record for June.
An official electronic reading will not be available until Monday morning.
More triple-digit temperatures in the forecast and an outage at a generating unit that supplies power to Northern California were responsible for triggering the Flex Alert, Cal-ISO spokesman Steven Greenlee said.
The Flex Alert calls for voluntarily cutbacks on energy use until after 6 p.m. to prevent reserves from falling to emergency levels.
Valleys and inland areas have been baking since Friday when a high pressure system caused heat to rise and linger.
"The system is so strong that it's suppressing movement of cooler air to move in, causing heat to continuously build over an area," said Craig Shoemaker, a meteorologist at the weather service's Sacramento office. "A big concern is that overnight lows are quite warm, expecting only to drop to the 70s or even 80s in most locations. That causes problems because it prolongs heat all day."
In Southern California, six half-marathon runners were taken to the hospital Sunday for heat-related illnesses, said Pasadena Fire Department spokeswoman Lisa Derderian.
The race, which began at 6:30 a.m., took its toll toward the end on some participants, who stopped short of the finish line, she said.
Paramedics were scattered along the 13.1-mile route in Pasadena and evaluated dehydrated runners who did not need to be hospitalized. In a typical race, they would see more sprained ankles and falls than heat-related problems.
Marathon organizers urged runners on its website to stay hydrated and promised extra water and ice towels along the route. Buses with air conditioning were provided for runners to cool off.
The event was supposed to be a marathon, but was changed to a half-marathon long before the heat wave due to low turnout last year.
"If this was a full marathon, we would have definitely been responding to a lot more calls," Derderian said.
Hikers, bikers and dog walkers were scarce on typically busy trails in the Santa Monica Mountains above Los Angeles. At midday, two women and a panting German shepherd huddled in a rare sliver of shade along a fire road before striking out in the hot sun.
Atop San Vicente Peak, cyclist Jeff Disbrow, 49, of Santa Monica was clad in black and lathered in sweat as he took a break and refilled his water bottle.
"It's not the best day to be out here—unless you want to suffer," he said. "It's like Arizona."