Thousands of flights have been canceled or delayed on Monday, and more than 1 million customers are without power across 11 states, as severe storms target the East Coast.

Cities seeing major impacts at airports include Atlanta, New York, Baltimore, Washington, D.C., and Boston. The Federal Aviation Administration said thunderstorms are also expected to impact airports in Memphis, Dallas, Denver, Orlando, Tampa, Fort Lauderdale and Miami.

The FAA issued multiple ground stops Monday night for several airports on the East Coast, including JFK, Newark, Philadelphia, Atlanta and airports in the Washington, D.C., area.

In Washington, D.C., federal employees, including at the White House and the Pentagon, were instructed to leave work early Monday afternoon due to the weather.

The main threats Monday afternoon and evening are tornadoes and destructive winds.

A severe thunderstorm watch is in effect for parts of Alabama, Georgia, South Carolina, North Carolina and Tennessee. A tornado watch is stretching across 11 states including Tennessee, West Virginia, Pennsylvania and New Jersey.

As of 8:45 p.m. ET, more than 1 million customers were without power across 11 states, according to poweroutage.us. North Carolina had the most outages, with at least 262,000 customers without power, followed by Pennsylvania, with around 183,000 customers in the state without power.

By 6 p.m. ET, the storms were expected to slam Washington, D.C., and Baltimore, and near Philadelphia, Raleigh and Atlanta.

The severe weather is expected to be gone by Tuesday morning.

Over the weekend, stormy weather spawned tornadoes, triggered flash flooding, knocked out power and uprooted trees across the U.S.

There were more than 300 damaging storm reports from Colorado to Virginia. There were also 10 reported tornadoes — eight across Iowa, Illinois, Nebraska and Colorado on Saturday and two in Illinois on Sunday, according to the National Weather Service.

Storm damage was reported from Wichita, Kansas, through central Illinois and into Birmingham, Alabama.

Torrential rainfall led to dangerous flash flooding in parts of northeastern Missouri on Friday night, including in the town of Kahoka, where more than 6 inches of rain fell within six hours. Water rescues were reported in the area.

Golf ball-sized hail was reported in Loveland, Colorado, and Almena, Kansas, on Saturday.

More than 40,000 people were left without electricity in Alabama on Sunday as gusty winds up to 61 miles per hour brought trees crashing down on power lines.