The winter weather that has overwhelmed power grids, leaving millions without electricity in record-breaking cold has kept its grip on parts of the US on 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 poisoning from a car exhaust in their garage, while another died after flames spread from their fireplace.

The polar vortex is to blame. This is 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 its southward escapes longer and more frequent.

More than 100 million people live in areas covered by some type of winter weather warning, watch or advisory on Wednesday as yet another winter storm hits Texas and 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.

Nearly three million customers remained without power early on 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 was braced for more icy rain and possibly more snow.

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

The weather has threatened to affect the nation’s Covid-19 vaccination effort, with President Joe Biden’s administration saying delays in vaccine shipments and deliveries were likely.

After visiting Milwaukee on Tuesday, Mr Biden said the weather was as “cold as the devil up there”.

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

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

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

“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,” Ms Harris said.

The situation in Texas drew attention at the International Energy Forum on Wednesday, 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,” Mr Barkindo said.

Travel remains ill-advised in much of the nation, with highways treacherous and thousands of flights cancelled. Some of the fatalities involve people dying in their cars in subfreezing temperatures.

Many school systems have delayed or cancelled face-to-face classes.

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

Authorities said a fire that killed three young children and their grandmother in the Houston area was likely to have spread from the fireplace they were using to keep warm.

In Oregon, authorities confirmed on Tuesday that four people had died in the Portland area from carbon monoxide poisoning.

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

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

“There is some hope on the horizon,” Mr Oravec said.