Subfreezing temperatures combined with failing electric power spurred the opening of heating centers from Oregon to Texas to West Virginia as the most unrelenting winter weather pattern in decades swept across the nation and left nearly three-quarters of the contiguous United States with some amount of snow cover.

Across the nation, 4.1 million customers were without power by Tuesday morning, 4 million of those customers in Texas alone. Another 223,000 customers were in the dark in Oregon and 202,000 across Oklahoma as temperatures dipped to levels of record-breaking cold.

By the afternoon, outages began declining, but still more than 3.7 million customers throughout Texas remained without power. The ongoing outages will cause an economic fallout bad enough to rival that of a significant hurricane season, AccuWeather Founder and CEO Dr. Joel N. Myers said.

The dropping temperatures and power outages prompted Texas Gov. Greg Abbott to announce Monday that the state would be deploying the National Guard to help conduct welfare checks and transport residents in need to one of the 135 warming centers.

As of late Monday night, the city of Dallas’ Office of Homeless Solution (OHS) and its partners were serving 629 people at shelters due to the inclement weather, including 475 who had sought shelter at the Kay Bailey Hutchinson Convention Center, a city spokesperson told AccuWeather via email.

The Kay Bailey Hutchinson Convention Center is not an overnight shelter, however, and while the city has had guests staying in hotel rooms to compensate for this, power outages did not spare even hotels. The guests were then relocated to the convention center.

Tuesday’s average high in Dallas is up to around 61 degrees Fahrenheit, but temperatures are forecast to dip and run a solid 20 degrees or more below average before Saturday when the cold snap is expected to break.

“The coldest of the cold is now, but the chill likely persists through the rest of this before any appreciably warming trend kicks off this weekend,” AccuWeather Meteorologist Matt Benz said. “The daytime highs are downright frigid, but low temperatures at night are even more dangerous with temperatures well below freezing through Friday night.”

At least 10 fatalities related to the winter weather have been reported in the Houston area, according to The Houston Chronicle, including an older woman and three of her grandchildren found in a house fire in Sugar Land, a city half an hour away from Houston. Firefighters found the mother outside of the home with burn injuries, and she is expected to survive.

Sugar Land Fire Department spokesman Doug Adolph said that they had been trying to keep warm with a fireplace, citing posts on the family’s social media accounts, as the neighborhood was without power.

The total number of fatalities also includes two men who were found dead along the Houston-area roadways, according to The Associated Press. While the causes of death were still under investigation, officials said the temperatures were likely to blame.

“We’re living through a really historic event going on right now,” Jason Furtado, a professor of meteorology at the University of Oklahoma, told the AP, referencing the winter storm warnings across Texas and the extent of the freezing temperatures.