Residents working to clear a fallen tree after heavy storms in Knoxville, Tenn., on Monday | Saul Young/News Sentinel, via USA Today Network

• Update: As the powerful storm moved on to New England, thousands were still without power.

A “complicated and active” storm system swept across the Eastern United States on Monday evening, delivering widespread thunderstorms that killed at least two people, grounded thousands of flights and left more than a million homes and businesses without power.

The line of storms barreled through a stretch from Georgia to New York, downing power lines, sending trees crashing into homes and tearing roofs from buildings, according to preliminary reports from the National Weather Service.

The Federal Aviation Administration ordered airports to ground flights along a busy travel corridor that links major U.S. cities, including Atlanta, New York and Washington, leading to thousands of flights being delayed or canceled.

At least one tornado was confirmed, just after 5:30 p.m. in the village of McGraw, about 30 miles south of Syracuse, N.Y., but caused minimal damage, Mayor Allan Stauber said on Tuesday afternoon. He noted that tree limbs were down, but he had not received any reports of major structural damage. A local TV station reported there was one roof blown off a structure.

In Florence, Ala., a 28-year-old man died after he was struck by lightning in a parking lot in the city, about 60 miles west of Huntsville, local police said. And in Anderson, S.C., a 15-year-old boy was killed when a large tree fell and struck him, according to local fire officials.

In Pennsylvania, a person was injured when a tree fell on the car they were in, according to the preliminary reports. In Virginia, officials rescued a woman who was trapped inside her home after a tree fell on it.

Some of the worst-hit areas were along the Mason-Dixon line and the southern Appalachians, said David Roth, a meteorologist with the Weather Service.

He cautioned that while the line of storms, known as a bow echo, had moved off the New Jersey coast, there was still a risk of “excessive rainfall” overnight into Tuesday across parts of upstate New York and Vermont. There, he said, “the heavy rainfall may just be beginning,” noting that the region could expect several inches.

In Cambridge, Md., several inches of rain caused flash flooding that stranded more than a dozen people in their cars on deluged roads, Chief Justin Todd of the Cambridge Police Department wrote by email. No injuries or deaths had been reported, he said, noting that several streets were closed as the police worked with local officials to get debris cleared from the roads.

Rob Kramer, Jr., a Dorchester County councilman, said that while the water was receding, “several roads” remained flooded.

As of early Tuesday, nearly 400,000 homes and businesses from Georgia to Pennsylvania remained without power, according to poweroutage.us.

Scientists say that climate change has supercharged storms in recent years, making hurricanes more intense and heavy downpours more frequent. Droughts, floods, wildfires and heat waves have all become increasing threats.

Where extreme rainfall is a concern, so is the risk of catastrophic flooding. But the rising temperatures make the problem worse: They allow the air to hold more moisture, leading to more intense and sudden rainfall.