A winter storm dropping snow and ice also sent temperatures plunging across the southern Plains.

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 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,” the council tweeted as it urged residents to reduce electricity use. ERCOT manages the flow of electric power in the state.

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

More than 1.3 million customers were in the dark as of 3 a.m. local time, according to poweroutage.us, a utility tracking site.

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.

Nearly 120 crashes, including a 10-car pileup on I-45, were reported Sunday, Houston Fire Chief Samuel Peña tweeted.

Significant ice and up to 12 inches (30 centimeters) of snow were expected across parts of the southern Plains into Monday, said Marc Chenard, a meteorologist with the National Weather Service's Weather Prediction Center.

“Typically, we just don't have quite this much cold air in place that far south,” Chenard said.

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, and had warned on Saturday: “All of Texas is facing an extremely dangerous winter storm.”

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.

In a statement Sunday night, President Joe Biden also declared an emergency in Texas and ordered federal assistance to aid state and local response efforts. The declaration allows the Department of Homeland Security and the Federal Emergency Management Agency to coordinate disaster relief efforts and provide assistance, equipment and resources to those affected by the storm.

More than 760 flights were canceled at Dallas-Fort Worth International Airport, and at Dallas Love Field most of the nearly 200 flights for Southwest Airlines, the airport’s main carrier, were canceled on Sunday.

American Airlines said about 345 of their flights were canceled at DFW Airport, its hub, by early Sunday afternoon. The airline said the storm was also affecting their flights across the region, with operations reduced and canceled at airports across Texas, Oklahoma and Arkansas.

Officials were discouraging travel in the wintry conditions.

The Oklahoma Highway Patrol said a portion of the Turner Turnpike was shut down due to a multi-vehicle accident, while the Oklahoma Department of Transportation said the southern corridor of Interstate 35 was mostly snow packed in the left lane and conditions were expected to deteriorate.

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 Memphis, Tennessee, snow started falling Sunday afternoon, and while main roads were still passable, lines formed during the day at grocery stores as people rushed to stock up.

In Mississippi, sleet in Jackson and other central parts of the state left roads and bridges slick. Bill Parker, a National Weather Service meteorologist in Jackson, said up to three-quarters of an inch (2 centimeters) of ice could accumulate in central Mississippi, bringing the possibility of power outages or falling tree limbs.

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

Meanwhile, in the Pacific Northwest, tens of thousands of people were without power after a winter storm blanketed the region with ice and snow and made travel treacherous.

Through the wintry conditions Sunday, florists stayed busy for Valentine's Day.

In Fort Worth, Texas, Gordon Boswell Flowers’ general manager said delivery drivers worked to wrap up deliveries before the weather worsened.

“It is icy and snowing and they’re calling for more snow,” Cheri Kirkman said.

Despite the weather, she said they still had some people coming in to pick up gifts. “We’ve got plenty all made up, ready to go,” Kirkman said.

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Associated Press journalists Julie Walker in New York City; John Raby in Charleston, West Virginia; Adrian Sainz in Memphis, Tennessee; and Emily Wagster Pettus in Jackson, Mississippi, contributed to this report.