Photo: The Canadian Press

A woman walks through falling snow in San Antonio, Texas, Sunday.

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 cancelled 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 to about -5 C 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 cancelled across the country, about 1,600 of them at Dallas/Fort Worth International and Bush Intercontinental airports in Texas. At DFW, the temperature was -15 C.

Accumulating ice was possible across eastern Louisiana, Mississippi, central Tennessee, Kentucky and over into the West Virginia and Ohio border region, Oravec said.

Up to 30 centimetres 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.