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California residents are being asked to reduce their electricity use for a second day Thursday to avert rolling blackouts amid a heat wave that threatens to strain the state’s power grid.

A Flex Alert has been extended into Thursday, from 4 p.m. to 9 p.m., by the California Independent System Operator, the nonprofit that runs the grid. It follows another Flex Alert in effect for the same hours on Wednesday.

The grid operator said it’s calling for voluntary conservation as much of California and the U.S. West are mired in a heat wave while energy supply margins are tight. The ISO expects a spike in demand from air conditioning use, which can strain the grid in the evening when solar energy isn’t being harvested.

In Southern California, temperatures are expected to be 5 to 10 degrees warmer than normal through the weekend, mostly in inland areas, forecasters say.

For the Los Angeles area, high temperatures are expected to reach 96 to 108 degrees during a day while nights will stay warm, with lows between 70 and 80 degrees, according to the National Weather Service, which advised residents to “stay cool, drink water and limit time in the heat.”

A heat advisory is in effect through 9 p.m. Friday for the Inland Empire, the Santa Ana Mountains and foothills and mountain communities in San Bernardino and Riverside counties.

Heat is forecast to be in the upper 80s to upper 90s in the mountain areas, while high temperatures are expected to reach up to 103 degrees in cities like Riverside, San Bernardino, Ontario, Moreno Valley, Fontana, Rancho Cucamonga and Corona.

Low humidity is also expected to bring elevated fire danger, although there’s a slight chance of storms for Thursday, the weather service said.

Flex Alerts are a call to consumers to voluntarily conserve energy when demand for power could outstrip supply, which generally occurs during heat waves when electrical demand is high. Such conservation would help ease the strain on the grid during the crucial evening hours when solar energy is diminished or no longer available, the ISO said.

Before the Flex Alert takes effect Thursday and when solar energy is abundant, residents are encouraged to take these steps to be comfortable and help grid operators balance supply and demand:

  • Pre-cool your home by lowering the thermostat to 72 degrees
  • If you need to use your major appliances, do it before the Flex Alert is in effect, when solar energy is plentiful
  • Close blinds and drapes to keep the heat out of your home or apartment
  • Charge electronic devices and electric vehicles so there’s no need to do it later

During the Flex Alert period, consumers are encouraged to:

  • Set thermostats to 78 degrees or higher, or use fans to cool the home, if your health permits
  • Avoid using major appliances like dishwashers, clothes washers and dryers
  • Turn off all unnecessary lights
  • Unplug unused items

Rolling blackouts have become a method for utility companies in the state to intentionally turn off the power when it gets too windy in dry summer months to prevent power lines from toppling and starting wildfires. Such blackouts in August 2020 were the first in nearly two decades caused by an energy shortage.