AUSTIN, Texas >> 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 today.

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 today 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.

Nearly 3 million customers remained without power early today 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.

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.

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 today 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.

The situation in Texas drew attention at today’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.

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.