▶ Watch Video: Extreme heat, severe weather continuing across U.S.

Tornado watches and warnings were posted along the East Coast on Monday as meteorologists warned millions of people to brace for severe thunderstorms, damaging winds and large hail. 

The National Weather Service forecast “very active” weather on the East Coast from late afternoon into the evening, with severe weather, heavy rains and flash flooding possible from the Southeast to the Mid-Atlantic and up into parts of the Northeast. 

Heavy rains and flash floods are a concern for eastern New York into New England Monday evening into Tuesday. Ping pong ball-sized hail was possible in some areas. 

The greatest threat is across the southern and central Appalachians to the Mid-Atlantic region, according to the weather service. Strong storms knocked down trees and caused other damage in Tennessee earlier.

The National Weather Service forecast for the East Coast shows extreme weather.

National Weather Service

“Some perspective: This is the first moderate risk for severe weather in at least 10 years for much of our forecast area,” the NWS service out of the Baltimore-Washington area said on social media.

Federal employees in Washington, D.C., were told to go home early because of the extreme weather forecast and federal offices were set to close at 3 p.m., according to the U.S. Office of Personnel Management. 

With the threat of severe weather ongoing, there have been a growing number of flight delays and cancellations, according to the flight-tracking website FlightAware. The Federal Aviation Administration said it was re-routing flights around the storms as much as possible. 

“Soon we will likely have to pause departures in and out of East Coast airports including NYC, Philly, DC, Charlotte and Atlanta,” the agency said late Monday afternoon.

Parts of the region have already dealt with extreme rain and flooding in recent weeks. Several people died in Pennsylvania during flash flooding last month. There was also deadly flooding in New York.

New York City’s Emergency Management department on Monday activated the city’s Flash Flood Plan in anticipation of the weather system.