AUSTIN, Texas (AP) — A frigid blast of winter weather across the U.S. plunged Texas into an unusually icy emergency Monday that knocked out power to more than 2 million 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. 

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Temperatures nosedived into the single-digits as far south as San Antonio, and homes that had already been without electricity for hours had no certainty about when the lights and heat would come back on, as the state's overwhelmed power grid throttled into rotating blackouts that are typically only seen in 100-degree Fahrenheit (38-degree Celsius) summers. 

The storm was part of a massive system that brought snow, sleet and freezing rain to the southern Plains and was spreading across the Ohio Valley and to the Northeast. The Southwest Power Pool, a group of utilities across 14 states, called for rolling outages because the supply of reserve energy had been exhausted. Some utilities said they were starting blackouts, while others urged customers to reduce power usage.

"We're living through a really historic event going on right now," said Jason Furtado, a professor of meteorology at the University of Oklahoma, pointing to all of Texas under a winter storm warning and the extent of the freezing temperatures. 

In Houston, where county leaders had warned that the freeze could create problems on the scale of massive hurricanes that slam the Gulf Coast, one electric provider said power may not be restored to some homes until Tuesday. 

"This weather event, it's really unprecedented. We all living here know that," said Dan Woodfin, senior director of system operations at the Electric Reliability Council of Texas. He defended preparations made by grid operators and described the demand on the system as record-setting. 

"This event was well beyond the design parameters for a typical, or even an extreme, Texas winter that you would normally plan for. And so that is really the result that we're seeing," Woodfin said. 

The largest grocery store chain in Texas, H-E-B, closed locations around Austin and San Antonio, cities that are unaccustomed to snow and have little resources to clear roads. The slow thaw and more frigid lows ahead was also taking a toll on Texas' distribution of COVID-19 vaccines. 

State health officials said Texas, which was due to receive more than 400,000 additional vaccine doses this week, now does not expect deliveries to occur until at least Wednesday. 

But with doses already in-hand expiring, Rice University on Monday abruptly began offering vaccines on its closed Houston campus. Harris Health System told the school it had about 1,000 vaccines that "were going to go to waste" and asked if the school could find takers, said Doug Miller, a university spokesman. 

 "The window was just a couple hours. They have to take care of it quickly," Miller said. 

Caught without enough groceries on hand, Lauren Schneider, a 24-year-old lab technician, walked to a Dallas grocery store near her home Monday morning dressed in a coat, hat and face mask. Schneider said she didn't feel comfortable driving with the roads covered in snow and ice. She said she hadn't seen a serious snowfall in Dallas since her childhood.

"I really didn't think it's would be this serious," said Schneider. 

Teresa and Luke Fassetta, trundling through the snow carrying grocery bags, said the store lost power while they were shopping. The couple said they lost power overnight, then got it back around 9 a.m., and they were hoping it would still be on when they arrived home. If not, Teresa said, "we just have a bunch of blankets and candles and two cats to keep us warm."

Several cities in the U.S. saw record lows as Arctic air remained over the central part of the country. In Minnesota, the Hibbing/Chisholm weather station registered minus 38 degrees Fahrenheit (minus 39 degrees Celsius), while Sioux Falls, South Dakota, dropped to minus 26 Fahrenheit (minus 26 degrees Celsius).

In Kansas, where wind chills dropped to as low as minus 30 degrees Fahrenheit (minus 34 degrees Celsius) in some areas, Gov. Laura Kelly declared a state of disaster.

Most government offices and schools were closed for Presidents Day, and authorities pleaded with residents to stay home. Louisiana State Police reported that it had investigated nearly 75 weather-related crashes caused by a mixture of snow, sleet and freezing rain in the past 24 hours.

"We already have some accidents on our roadways," Kentucky Gov. Andy Beshear said during a morning news conference. "It is slick and it is dangerous."

Air travel was also affected. 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.

The storm arrived over a three-day holiday weekend that has seen the most U.S. air travel since the period around New Year's. More than 1 million people went through airport security checkpoints on Thursday and Friday. However, that was still less than half the traffic of a year ago, before the pandemic hit with full force.

The southern Plains 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.

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Associated Press journalists David Koenig in Dallas, Rebecca Reynolds Yonker in Louisville, Ky., Kate Brumback in Atlanta, Margaret Stafford in Liberty, Mo., and Amy Forliti in Minneapolis contributed to this report.