WASHINGTON (AP) — At least two people died, thousands of U.S. flights were canceled or delayed, and more than 1.1 million homes and businesses lost power Monday as severe storms, including hail and lightning, moved through the eastern U.S.

The National Weather Service issued a tornado watch for the greater D.C. area, lasting until 9 p.m. A special Weather Service statement warned, “There is a significant threat for damaging and locally destructive hurricane-force winds, along with the potential for large hail and tornadoes, even strong tornadoes.”

The storms’ spread was massive, with tornado watches and warnings posted across 10 states from Tennessee to New York. The National Weather Service said more than 29.5 million people were under a tornado watch Monday afternoon.

In Anderson, South Carolina, a 15-year-old boy who arrived at his grandparent’s house during the storm was struck and killed when a tree fell on him as he got out of a car, according to the Anderson County Office of the Coroner.

In Florence, Alabama, police said a 28-year-old man was struck by lightning and died, WAAY-TV reported.

By Monday night, more than 2,600 U.S. flights had been canceled and nearly 7,900 delayed, according to flight tracking service FlightAware. Many cancellations were at Hartsfield-Jackson Atlanta International Airport, which was digging out from disruptions caused by Sunday storms.

The Federal Aviation Administration said it was rerouting planes around storms heading to the East Coast.

The White House pushed up by 90 minutes President Joe Biden’s departure on a four-day trip that’s taking him to Arizona, New Mexico and Utah. The White House also canceled a back-to-school cybersecurity event that was to feature first lady Jill Biden, who is a teacher, Education Secretary Miguel Cardona, Homeland Security Secretary Alejandro Mayorkas and school administrators, educators and education technology providers from around the country.

The Office of Personnel Management announced Monday that all non-emergency employees would have to depart before 3 p.m., when all federal offices closed.

“This does look to be one of the most impactful severe weather events across the Mid-Atlantic that we have had in some time,” National Weather Service meteorologist Chris Strong said in a Facebook live briefing.

The storms were expected to strike major population areas in late afternoon and early evening, prompting federal workers to be sent home early so they wouldn’t be in their cars amid wind, hail and tornadoes.

Strong advised residents: “Have yourself in a strong shelter. Be at home or be at work.”

The storms postponed a Major League Baseball game between the Phillies and the Washington Nationals in Philadelphia, and in Maryland, the National Weather Service issued a flash flood warning into Tuesday after 4 inches (10.2 centimeters) of rain fell in a short amount of time.

By early evening, more than 1.1 million customers were without power across Alabama, Georgia, South Carolina, North Carolina, Maryland, Delaware, New Jersey, Pennsylvania, Tennessee, West Virginia and Virginia — all states along the storm system’s path, according to poweroutage.us. The Knoxville Utilities Board tweeted that the damage across its service area in Tennessee was “widespread and extensive” and will likely take several days to repair.

Trees and power lines were toppled in multiple states, falling into roads and some homes, news outlets reported.

A row of utility poles was toppled in Westminster, Maryland, WJLA-TV reported.

In Hockessin, Delaware, at least one residence had the roof ripped off, 6ABC-TV reported.

“We saw the clouds coming and could hear a rumbling in the distance,” said Tom Tomovich, whose home was damaged. “We went into the house and we were on the first floor, and before we could blink an eye the winds just came right through the back of our house.”

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Collins reported from Columbia, South Carolina. Associated Press writer Darlene Superville in Washington and AP Airlines Writer David Koenig contributed to this report.