WASHINGTON—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.