LOS ANGELES (AP) — Californians were urged to voluntarily cut their electricity use Thursday in a rare mid-winter conservation alert, after frigid weather across the U.S. and Canada caused a shortage of natural gas at Southern California power plants.

“While the natural gas shortage is only impacting Southern California power plants, statewide electricity and gas conservation will help free up both electricity and gas supplies for Southern Californians,” the California Independent System Operator, which runs the state’s power grid, said in a statement.

Requests for Californians to curtail their power use typically occur in summer, when temperatures soar and air conditioners roar, especially across Southern California.

The so-called Flex Alert, in which residents are asked to turn off unneeded lights, avoid using large appliances or equipment, and turn off electrically powered heaters, was set to expire at 10 p.m. Thursday.

It wasn’t immediately clear if the conservation request would extend beyond a single day.

Stephanie McCorkle, a spokeswoman for the grid operator, said Southern California has become increasingly dependent on natural gas-fired plants since the decision last year to shutter the troubled San Onofre nuclear power plant, which is located between Los Angeles and San Diego.

When it was operating, the twin-reactor San Onofre plant produced enough power for 1.4 million homes.

According to 2010 California Energy Commission research, 53 percent of the power generated in the state comes from natural gas.

Record amounts of natural gas are being burned for heat and electricity across North America.

Research firm Bentek Energy said in a statement Thursday that domestic natural gas production dropped about 1 percent in January from the previous month. That doesn’t include Alaska or Hawaii.

“The recent and persistent cold in the U.S. Northeast and Midwest regions affected overall production this month, given that wells can freeze during very cold weather,” Jack Weixel, Bentek’s director of energy analysis, said in a statement.

Bentek analyst Luke Jackson said gas supplies entering Southern California pipelines have been considerably lower the past two days because of high gas prices and strong demand in Texas, the Pacific Northwest and the Rocky Mountain states, which has crimped gas supplies for Southern California.

The Electric Reliability Council of Texas, which operates most of that state’s electric grid, asked people to reduce electric use until noon Friday. Peak demand Thursday morning exceeded 57,000 megawatts and could break the record of 57,277 megawatts before Texas’ cold temperatures subside, the council said in a news release.

“We are expecting cold weather to continue through tomorrow morning’s high demand period, and some generation capacity has become unavailable due to limitations to natural gas supplies,” said Dan Woodfin, the council’s director of system operations.