DALLAS (NewsNation Now) — A frigid blast of winter weather across the U.S. plunged Texas into an unusually icy emergency that knocked out power to millions of people and shut down grocery stores and dangerously snowy roads.

The worsening conditions halted the delivery of COVID-19 vaccine shipments and left some Texas providers scrambling to find takers for doses expiring within hours.

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 around Houston. The outages began as rotating outages but ERCOT had to switch to controlled outages — which can be much, much longer — due to the magnitude of the demand.

“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.

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

NewsNation affiliate KXAN reports some Texans without power Monday afternoon could be forced to remain without it through Tuesday, according to an emergency update from Oncor.

“The Texas power system is currently facing an unprecedented shortfall of electric generation,” Oncor said in a statement. “The Electric Reliability Council of Texas (ERCOT) has requested Oncor and utilities across the state to implement controlled power outages to reduce high demand and protect the integrity of the electric grid.

All Texas electric utilities could face similar extended outages, ERCOT said in a news briefing Monday morning.

“That’s what they did in my neighborhood … my understanding is that the blackouts are because of that concept, that it was to be a rolling blackout,” Travis County Judge Andy Brown told NewsNation affiliate KXAN. “And then the power situation was so grave at the state level that they weren’t able to follow through on that plan for some reason and they weren’t able to turn some of the power back on in the way they had anticipated.”

Brown did the live interview from his car to stay warm because the power had gone out in his home.

“These outages will continue until there’s sufficient generation being able to be brought back online to meet the demands on the system,” said Dan Woodfin, ERCOT’s senior director of system operations. “At this time we anticipate that we’ll need to continue these control outages at some level for the rest of today and at least first part of [Tuesday], perhaps all day tomorrow.”

More than 4.1 million customers in Texas were in the dark as of 4:30 a.m. EST Tuesday, according to poweroutage.us, a utility tracking site.

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.

Texas Gov. Greg Abbott tweeted Monday that the Public Utilities Commission said “about 500,000 residential customers are having power restored at this time. They said to expect even more this evening.”

Abbott maintains the power grid is not broken, but said that parts of the grid had to be shut down, including natural gas and coal generators.

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.”

State leaders are urging Texans to stay off the roads. On Monday, the Texas Department of Transportation explained an expected storm overnight Tuesday could prolong the treacherous road conditions even longer.

Law enforcement reported two men were found dead along Houston-area roadways. Causes of death were pending, but officials said the subfreezing temperatures were likely to blame.

TxDOT’s Transportation Safety Directory Michael Lee said crews are working to try to clear as much snow and ice as possible before that second storm rolls in with more precipitation.

“Right now we have a window, it started this morning, some 24 hours depending on where you are in the state, 24 to 36 hours or so to get as much done as we can. And that’s what we’ve been working on it and working to get everything just pushed out of the way keeping the major roadways open,” Lee explained.

After that storm, it could be days before roads are fully melted and clear across the state. Lee said warming temperatures beginning Thursday will begin to help.

“Our primary focus will be on the all the major interstates and the major corridors and we’ll go down to the local corridors and then then the rural roadways so you know, I would expect that we can have the major roadways open by Monday of the next week,” Lee said.

He said if you do have to drive, make a plan in case your car gets stuck. Lee also said drivers should check with local law enforcement agencies for safe routes, along with TxDOT’s website for any closures and conditions.

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.

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 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. Gov. 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.”

On Monday, Abbott and the Texas Military Department deployed National Guard across the state to conduct welfare checks. They will also help local authorities “in transitioning Texans in need to one of the 135 local warming centers” that have been established across the state.

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.

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

Houston Bush Airport said early Monday morning that the airfield would be closed until further notice because of ice accumulation,

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 of snow in central Oklahoma, and 4 to 8 inches 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 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 of ice or up to 8 inches of snow by Tuesday.

Meanwhile, in the Pacific Northwest, more than 200,000 people were without power after a winter storm blanketed the region with ice and snow and made travel treacherous.

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 reporting to this story. The Associated Press contributed to this report.