People line up to enter Costco in Pflugerville, Texas, Tuesday morning, Feb 16, 2021. Anger over Texas' power grid failing in the face of a record winter freeze mounted Tuesday as millions of residents in the energy capital of the U.S. remained shivering with no assurances that their electricity and heat —out for 36 hours or longer in many homes—would return soon or stay on once it finally does. (Ricardo B. Brazziell/Austin American-Statesman via AP)

Winter weather that has overwhelmed power grids unprepared for climate change and left millions without electricity in record-breaking cold kept its grip on the nation's midsection Wednesday.

At least 20 people have died, some while struggling to find warmth inside their homes. In the Houston area, one family succumbed to carbon monoxide from car exhaust in their garage; another perished as they used a fireplace to keep warm.

Blame the polar vortex, a weather pattern that usually keeps to the Arctic, but is increasingly visiting lower latitudes and staying beyond its welcome. Scientists say global warming caused by humans is partly responsible for making the polar vortex's southward escapes longer and more frequent.

More than 100 million people live in areas covered Wednesday by some type of winter weather warning, watch or advisory, as yet another winter storm hits Texas and other parts of the southern Plains, the National Weather Service said.

Utilities from Minnesota to Texas and Mississippi have implemented rolling blackouts to ease the burden on power grids straining to meet extreme demand for heat and electricity as record low temperatures were reported in city after city. In Mexico, rolling blackouts Tuesday covered more than one-third of the country after the storms in Texas cut the supply of imported natural gas.

Howard and Nena Mamu eat dinner at their home in the Glenwood neighborhood in Hutto, Texas, Tuesday, Feb 16, 2021. Anger over Texas' power grid failing in the face of a record winter freeze mounted Tuesday as millions of residents in the energy capital of the U.S. remained shivering with no assurances that their electricity and heat —out for 36 hours or longer in many homes—would return soon or stay on once it finally does. (Ricardo B. Brazziell/Austin American-Statesman via AP)

Nearly 3 million customers remained without power early Wednesday in Texas, Louisiana and Mississippi, more than 200,000 more in four Appalachian states, and nearly that many in the Pacific Northwest, according to poweroutage.us, which tracks utility outage reports.

The latest storm front was predicted to bring snow and ice to East Texas, Arkansas and the Lower Mississippi Valley before moving to the northeast on Thursday. Winter storm watches were in effect from Baltimore to Boston, and Texas braced for more icy rain and possibly more snow.

"There's really no letup to some of the misery people are feeling across that area," said Bob Oravec, lead forecaster with the National Weather Service.

The weather has threatened the nation's COVID-19 vaccination effort. President Joe Biden's administration said delays in vaccine shipments and deliveries were likely.

People wait in a long line to buy groceries at H-E-B on South Congress Avenue during an extreme cold snap and widespread power outage on Tuesday, Feb. 16, 2021, in Austin, Texas. (Jay Janner/Austin American-Statesman via AP)

The worst U.S. power outages by far have been in Texas, where officials requested 60 generators from the Federal Emergency Management Agency and planned to prioritize hospitals and nursing homes. The state opened 35 shelters to more than 1,000 occupants, the agency said.

Texas' power grid manager, the Electric Reliability Council of Texas, said electricity had been restored to 600,000 homes and businesses by Tuesday night but that 2.7 million households were still without power.

Blackouts lasting more than an hour had begun before dawn Tuesday in and around Oklahoma City, stopping electric-powered space heaters, furnaces and lights just as temperatures hovered around minus 8 degrees (minus 22 degrees Celsius). Oklahoma Gas & Electric urged users to set thermostats at 68 degrees Fahrenheit (20 degrees Celsius), avoid using major electric appliances and turn off lights or appliances not in use.

This aerial photo shows the devastation Tuesday Feb. 16, 2021, in the Ocean Ridge Plantation area of Brunswick County, N.C. following a tornado. (Ken Blevins/The Star-News via AP)

Entergy imposed rolling blackouts Tuesday night in Arkansas, Louisiana, Mississippi and Southeast Texas at the direction of its grid manager, the Midcontinent Independent System Operator, "as a last resort and in order to prevent more extensive, prolonged power outages that could severely affect the reliability of the power grid," according to a statement from the New Orleans-based utility.

The Southwest Power Pool, a group of utilities covering 14 states, said the blackouts were "a last resort to preserve the reliability of the electric system as a whole."

Vice President Kamala Harris addressed those who had lost power during a live interview Wednesday on NBC's "Today."

"I know they can't see us right now because they're without electricity, but the president and I are thinking of them and really hope that we can do everything that is possible through the signing of the emergency orders to get federal relief to support them," Harris said.

Customers use the light from a cell phone to look in the meat section of a grocery store Tuesday, Feb. 16, 2021, in Dallas. Even though the store lost power, it was open for cash only sales. (AP Photo/LM Otero)

The situation in Texas drew attention at Wednesday's International Energy Forum, including messages of support from Saudi Arabia's energy minister and OPEC Secretary General Mohammed Barkindo.

"As the extreme weather in Texas has shown, we cannot take energy security for granted, even in a country like the United States," Barkindo said at the forum, which was held virtually.

Travel remains ill-advised in much of the United States, with roadways treacherous and thousands of flights canceled. Many school systems delayed or canceled face-to-face classes.

But even staying home can be hazardous in places without power.

Authorities said a fire that killed three young children and their grandmother in the Houston area likely spread from the fireplace they were using to keep warm. In Oregon, authorities confirmed Tuesday that four people died in the Portland area of carbon monoxide poisoning.

  • Power lines are shown Tuesday, Feb. 16, 2021, in Houston. More than 4 million people in Texas still had no power a full day after historic snowfall and single-digit temperatures created a surge of demand for electricity to warm up homes unaccustomed to such extreme lows, buckling the state's power grid and causing widespread blackouts. (AP Photo/David J. Phillip)

  • Crews with Tupelo Public Works begin to clear the intersection of South Green and Eason Blvd., Monday night, Feb. 15, 2021, as they work through the night to make the roads safe for travel in Tupelo, Miss. (Thomas Wells/The Northeast Mississippi Daily Journal via AP)

  • Ivan Gonzales, left, works with his brother-in-law Gabriel Martinez to assist a motorist using a carpet up a hill along the snow-covered Cherrywood Road in Austin, Texas, on Tuesday, Feb. 16, 2021. The statewide freeze has resulted in many roads to become layered with ice and snow making several impassible and hazardous to both drivers and pedestrians.. (Bronte Wittpenn/Austin American-Statesman via AP)

  • Pastor Gavin Rogers, left, tries to convince people to come to his warming shelter to escape sub-freezing temperatures, Tuesday, Feb. 16, 2021, in San Antonio. (AP Photo/Eric Gay)

  • A homeless man Tuesday, Feb. 16, 2021, sleeps at the Chicago Transit Authority's Clark & Dearborn bus station, the morning after a snowstorm dumped up to 18 inches in the greater Chicago area. (AP Photo/Charles Rex Arbogast)

  • City of Richardson worker Kaleb Love breaks ice on a frozen fountain Tuesday, Feb. 16, 2021, in Richardson, Texas. Temperatures dropped into the single digits as snow shut down air travel and grocery stores. (AP Photo/LM Otero)

  • Ashely Archibad sings with her children, Avett, from left, 8, Cohen, 4, Luella, 10, and Sanders, 6, while their father Brett pays his guitar at their home in the BlackHawk neighborhood in Pflugerville, Texas, Tuesday, Feb 16, 2021. Anger over Texas' power grid failing in the face of a record winter freeze mounted Tuesday as millions of residents in the energy capital of the U.S. remained shivering with no assurances that their electricity and heat —out for 36 hours or longer in many homes—would return soon or stay on once it finally does. (Ricardo B. Brazziell/Austin American-Statesman via AP)

  • Brett Archibad tries to entertain his family as they attempt to stay warm in their home the BlackHawk neighborhood in Pflugerville, Texas, Tuesday, Feb 16, 2021. Anger over Texas' power grid failing in the face of a record winter freeze mounted Tuesday as millions of residents in the energy capital of the U.S. remained shivering with no assurances that their electricity and heat —out for 36 hours or longer in many homes—would return soon or stay on once it finally does. (Ricardo B. Brazziell/Austin American-Statesman via AP)

  • Christine Chapman, center, sets down an empty canister to exchange for a full propane tank from Robert Webster, left, outside a grocery store Tuesday, Feb. 16, 2021, in Dallas. Even though the store lost power, it was open for cash only sales. Chapman said she has been without power for two nights and is using the propane to keep warm. (AP Photo/LM Otero)

  • People seeking shelter from below freezing temperatures rest inside a church warming center Tuesday, Feb. 16, 2021, in Houston. More than 4 million people in Texas still had no power a full day after historic snowfall and single-digit temperatures created a surge of demand for electricity to warm up homes unaccustomed to such extreme lows, buckling the state's power grid and causing widespread blackouts. (AP Photo/David J. Phillip)

At least 13 children were treated for carbon monoxide poisoning at Cook Children's Medical Center in Fort Worth and one parent died of the toxic fumes, hospital officials said.

In Texas, at least, temperatures were expected to rise above freezing by the weekend.

"There is some hope on the horizon," Oravec said.