INDIANAPOLIS >> People in the central U.S. headed into the July Fourth weekend facing smoky haze, high temperatures and dealing with the aftermath of powerful derecho winds that caused widespread damage and left hundreds of thousands of residents without power.

Utility crews were scrambling today to restore electricity after a storm system moved across Illinois and Indiana on Thursday packing winds that sometimes reached more than 70 miles per hour.

The storm damaged trees and buildings in the central parts of both states from the Mississippi River to the Indianapolis area. Utility companies faced the challenge of trying to replace electrical lines entangled in downed trees ahead of more expected thunderstorms and temperatures climbing to around 90 degrees Fahrenheit.

“We’re seeing a large number of broken poles, trees and powerlines, spans of wire down,” said Angeline Protogere, a spokeswoman for Duke Energy in Indiana.

Some communities in central Illinois and western Indiana declared disaster emergencies to limit traffic on roads for utility and cleanup crews to work. Utility companies reported that more than 250,000 homes and businesses were without electricity this morning.

The National Weather Service said the storm was a derecho, which is often described as an inland hurricane because of its line of strong winds stretching for hundreds of miles.

“We had damage all the way from northeast Kansas, all the way down into Kentucky and across Indiana,” said John Bumgardner, a meteorologist with the National Weather Service in Illinois.

In the South, a dangerous heat wave that has been blamed for the deaths of at least 14 people was expected to last into the weekend in some areas. Forecasters warned that heat indexes could rise above 110 degrees Fahrenheit (43 degrees Celsius) and an excessive heat warning remained in place today for parts of Tennessee, Arkansas and Mississippi.

In Memphis, Tennessee, city and county officials said relief efforts were focused on those who still had no power and air conditioning after strong storms Sunday that knocked down trees and power lines. About 10,000 homes and businesses still had no power on this morning, according to the local utility, Memphis Light, Gas and Water.

“To all of those customers, I’m sorry for what you’re going through. I know how difficult it can be in the absence of a utility and a commodity that you rely on for your daily life to help you cook, clean, and stay cool,” said Doug McGowen, the utility’s president and CEO, during a news conference Thursday.

The storm in the Midwest did help clear the region’s air of smoke from Canadian wildfires that had prompted warnings for people to stay inside. The Environmental Protection Agency had listed many cities, including Chicago, Detroit, Indianapolis and Cleveland, Ohio, as having “very unhealthy air” earlier in the week.

The EPA warned today that parts of Pennsylvania, New York, New Jersey and Connecticut could experience “unhealthy” air conditions because of the wildfires in Quebec and northern Ontario.

“The primary concern is high concentrations of fine particle air pollution that is unhealthy, especially for sensitive groups such as people with respiratory disease, the elderly, or people with compromised health,” the agency said.

The Midwest might only have a brief respite from the Canadian smoke as another storm is poised to move through the region Sunday, meteorologist Bumgardner said.

“Behind that our winds will probably switch back to northerly, which theoretically could bring a little more smoke into the area,” Bumgardner said. “But that’s tough to predict more than a day or two out.”

Parts of the West are anticipating extremely hot, dry conditions forecast through the Fourth of July, raising concerns about the danger of fireworks shows and wildfires. Communities that canceled fireworks shows in recent years due to drought still hope to go forward Tuesday. Salt Lake City is replacing fireworks with a drone show.

Airline travelers got some relief today, with none of the weather-induced restrictions imposed earlier this week on planes landing and taking off at major airports in the Northeast.

Still, by midday on the East Coast more than 2,000 flights had been delayed and more than 300 others canceled — more than 200 of those on United Airlines, according to FlightAware.

If the trends hold up, United will have the most cancellations of any U.S. airline for a seventh straight day.