WASHINGTON (AP) — The Washington area on Monday braced for a looming forecast of destructively strong storms, including tornadoes, hail and lightning, and officials warned residents to prepare for the worst.

Rain began falling in the Washington shortly after 5 p.m. and the skies gradually turned an ominous dark gray, a precursor to the severe weather and mass power outages that were predicted. Officials warned residents to stay indoors and prepare for the worst.

The National Weather Service issued a tornado watch for the greater D.C. area, lasting until 9 p.m., as well as a flood warning extending through Tuesday morning. 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 posted across 10 states from Tennessee to New York. But the National Weather Service said the area of greatest concern centered in the Washington-Baltimore region.

By late Monday afternoon, more than 1,300 U.S. flights had been canceled and 5,500 delayed, according to FlightAware. More than a quarter of the 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 and warned it would likely start pausing flights in and out of the New York City area, Philadelphia, Washington, Charlotte and Atlanta.

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.

Also concerning forecasters was the timing of the storms. They 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.”

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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.