Thousands of US flights were cancelled or delayed Monday as forecasts warned of destructively strong storms, including tornadoes, hail and lightning, and residents were warned to stay indoors and prepare for the worst.

Rain began falling in the Washington area shortly after 5 pm, and the skies gradually turned an ominous dark gray, a precursor to the severe weather and mass power outages that were predicted.

The National Weather Service issued a tornado watch for the greater DC area, lasting until 9 pm, 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 and warnings posted across 10 states from Tennessee to New York. The National Weather Service said the area of greatest concern centered in the Washington-Baltimore region. afternoon, more than 1,300 US flights had been canceled and 5,500 delayed, according to flight tracking service 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. more than 580,000 customers were without power across Georgia, North Carolina, Maryland, Tennessee 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.

Dominion Energy reported nearly 15,000 people without power in Loudoun County, Virginia, west of D.C. And Pepco, a power company serving Washington and its suburbs, reported outages affecting more than 2,000 people.