Dangerously cold temperatures settled over a wide swath of the US and Canada on Saturday as a massive winter storm upended Christmas holiday travel and left millions without power or facing rolling blackouts.

Utility companies from Texas to New York City were urging customers to conserve power to protect supply as people turned up their thermostats in temperatures the National Weather Service describes as “life threatening.”

Meanwhile, thousands of flights were canceled or delayed on both Saturday and Sunday, according to FlightAware, and road travel has been treacherous, forcing some to cancel Christmas plans.

New York Governor Kathy Hochul, who has mobilized the National Guard to the state’s western Erie Country, the region hardest hit by the storm, said it “may go down as one of the worst in history.” Buffalo Niagara International Airport is closed at least through Monday morning, she told a news conference on Saturday. In New York City, Hochul toured flooded areas of the Rockaways in Queens and said she has asked the federal government to declare a disaster area.

Temperatures were 20 to 30 degrees Fahrenheit (11 to 17 C) below normal across much of the central and eastern US, the NWS said, and snow totals in parts of the Midwest have reached or are almost at record levels. Buffalo probably set a record with 77 inches (196 cm) of snowfall in part of the city from Thursday night until Saturday morning, with more on the way in the region through Saturday night, the weather agency said.

It’s just the latest example of extreme weather brought on by climate change wreaking havoc on power grids, halting travel and upending economies. This time, an extreme swerve in the jet stream was blamed.

Power grids were taxed by frigid temperatures, causing one of the most sprawling US power outages in years. Some 1.6 million homes and businesses from Texas to New England were without power early Saturday as the storm marched across the country, according to Poweroutage.us, which tracks utility websites. The number fell to almost 850,000 by late afternoon. North Carolina and Maine were hardest hit, accounting for nearly two-thirds the outages.

Consolidated Edison Inc. which supplies electric service in New York, parts of New Jersey and Pennsylvania, on Saturday evening urged customers to “conserve energy due to frigid weather and increased demand on interstate pipelines that bring natural gas into the New York City Metropolitan region.”

The call for conservation covers Con Edison’s 1.1 million natural gas and 3.5 million electric customers and its steam customers in Manhattan, a statement said. It will remain in effect until further notice.

The nation’s largest power grid, which stretches from Illinois to New Jersey and serves 65 million people, said it might be forced to employ rolling blackouts as the cold drove demand for electricity to almost unprecedented levels. The grid operator, PJM Interconnection LLC, urged customers to conserve.

“The possibility of rotating customer outages is real,” said PJM Senior Vice President of Operations Mike Bryson in a video posted on Twitter. “We are going to do everything we can to prevent that, but we think it’s important that consumers are ready in case we have to take that step.”

The US Energy Department declared a power emergency in Texas, citing a shortage of electricity as an Arctic winter blast causes power plants to fail.

In Canada, where every province and territory is under an emergency weather warning, more than 200,000 customers out of about 4.6 million were without power on Saturday, according to utility Hydro-Québec.

The Tennessee Valley Authority ordered rolling blackouts for the second straight day as demand for power soared while high winds knocked power plants offline. Duke Energy Corp. is undertaking rolling outages in North Carolina.

Nashville Mayor John Cooper asked the city’s NFL team, the Titans, to postpone its game against the Houston Texans. The game kicked off about an hour later than scheduled.

US natural gas production suffered its worst one-day drop in more than a decade on Friday as liquids froze in pipes, forcing wells to shut. Supplies of the heating and power-generation fuel across the continental US shrank by nearly 10 billion cubic feet, or roughly 10%, from the previous day as temperatures across key producing areas including in top supplier Texas fell below freezing, according to BloombergNEF data.

Meanwhile, domestic demand surged to the highest daily level since early 2019. Early pipeline nominations tracked by BNEF indicate that gas supplies may start to rebound Saturday while still trailing normal levels.

The storm moved over western Quebec by Saturday morning, keeping conditions in the US Northeast windy, said Marty Rausch, a meteorologist with the Weather Prediction Center. It will begin to lift out Sunday into Monday, when the central and eastern parts of the US will begin to warm up.

The storm, which forecasters described as a once-in-a-decade phenomenon because of its size and speed, left more than 200 million Americans — around 60% of the country — under some form of winter weather warning or advisory starting Friday. It achieved the status of a “bomb cyclone” as it swept east.

It also created “substantial disruptions” at FedEx Express hubs in Memphis and Indianapolis, potentially delaying holiday packages from arriving by Christmas, the shipping company said in a statement.