DALLAS (AP) — A winter storm dropping snow and ice also sent temperatures plunging across the southern Plains, prompting a power emergency in Texas a day after conditions canceled flights and impacted traffic across large swaths of the U.S.

Rotating power outages were initiated by the Electric Reliability Council of Texas, or ERCOT, early Monday morning, meaning hundreds of thousands went without electricity for short periods as temperatures fell into the teens near Dallas and 20s (about minus 5 degrees Celsius) around Houston.

“We urge Texans to put safety first,” ERCOT tweeted as it urged residents to reduce electricity use. The council manages the flow of electric power in the state.

“Traffic lights and other infrastructure may be temporarily without power,” the council said.

The council described the rotating outages as a “last resort to preserve the reliability of the electric system as a whole,” adding that utility transmission companies are tasked with determining how to reduce demand on the system.

“Every grid operator and every electric company is fighting to restore power right now,” ERCOT President and CEO Bill Magness said in a statement.

The rotating outages were expected to last throughout Monday morning.

Nearly 2.6 million customers in Texas were in the dark as of 8:20 a.m. CST, according to poweroutage.us, a utility tracking site.

Matt Varble in the Dallas suburb of Las Colinas told The Dallas Morning News his power had gone out a couple of times Monday morning. The second time, it went out about 3:30 a.m. and hadn’t returned as of 7 a.m.

“It’s starting to get very cold inside my house,” Varble told the newspaper. “I lived in the north for a very long time and nothing like this has ever happened when I lived in New York, Ohio and Illinois.”

Around 5,000 Oklahoma Gas & Electric customers were without power overnight, and Entergy Arkansas logged about 3,000 outages. Both states have much smaller populations compared with Texas.

Officials in Houston had warned people to prepare for outages and hazardous roads — conditions similar to what residents might see in the wake of a Category 5 hurricane.

“There (have) been numerous reports of accidents from icing recently,” National Weather Service lead forecaster Bob Oravec said Monday. “I think there’s going to be a big threat today as the system pushes northeastward.”

By midmorning, 3,000 flights had been canceled across the country, about 1,600 of them at Dallas/Fort Worth International and Bush Intercontinental airports in Texas. At DFW, the temperature was 4 degrees Fahrenheit (-15 degrees Celsius) — 3 degrees (-16 degrees) colder than Moscow

Accumulating ice between a tenth and a quarter of an inch (0.25 and 0.6 centimeters) was possible across eastern Louisiana, Mississippi, central Tennessee, Kentucky and over into the West Virginia and Ohio border region, Oravec said.

Up to 12 inches (30 centimeters) of snow was expected across parts of the southern Plains into Monday, said Marc Chenard, a meteorologist with the National Weather Service’s Weather Prediction Center.

The region had been gearing up for the winter weather for the better part of the weekend. Texas Gov. Greg Abbott issued a disaster declaration for all of the state’s 254 counties. Abbott, Oklahoma Gov. Kevin Stitt and Arkansas Gov. Asa Hutchinson each activated National Guard units to assist state agencies with tasks including rescuing stranded drivers.

President Joe Biden also declared an emergency in Texas in a statement Sunday night. The declaration is intended to add federal aid to state and local response efforts.

The National Weather Service said Sunday that the forecast through early Tuesday calls for 8 to 12 inches (20 to 30 centimeters) of snow in central Oklahoma, and 4 to 8 inches (10 to 20 centimeters) in an area extending from eastern Texas to the Ohio Valley in the Northeast.

In Louisiana, police closed multiple bridges and parts of some interstates because of icy conditions around Baton Rouge. Notably, Interstate 10 between Baton Rouge and Lafayette was closed in both directions Monday morning because of ice accumulation that caused multiple crashes

In Memphis, Tennessee, snow started falling Sunday afternoon, and in Mississippi, sleet in Jackson and other central parts of the state left roads and bridges slick.

Parts of Kentucky and West Virginia still recovering from an ice storm last week were expected to get up to a quarter-inch (0.6 centimeters) of ice or up to 8 inches (20 centimeters) of snow by Tuesday.

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