More than 1,100 flights have been canceled Monday across the United States -- and more than 3,000 delayed -- as severe weather threatens many eastern states.

The most-impacted airport is Atlanta's Hartsfield-Jackson, which is also the world's busiest, according to tracking site FlightAware. As of Monday afternoon, almost 100 outbound flights have been canceled, and more than 250 have been delayed.

There are also significant issues at Baltimore's airport, where nearly one in five flights has been canceled, and New York's LaGuardia and John F. Kennedy airports where there are ongoing disruptions.

Delta Air Lines, headquartered in Atlanta, has the most cancellations among US airlines, according to the FlightAware figures. Delta canceled at least 280 flights, or 8 percent of its schedule, and delayed more than 500, or 15 percent of its schedule.

Nearly 700 Southwest flights have been delayed, according to FlightAware, and more than 180 have been cancelled.

CNN has reached out to Delta and Southwest for comment.

The Federal Aviation Administration reported it was slowing flights headed towards New York, Philadelphia, and Washington, D.C. area airports because of thunderstorms. It warned of impacts this afternoon to flights from Florida north to New York due to the storms. Around 120 million people in the Eastern US could be impacted by severe weather Monday.

While the weather may be to partly to blame, these delays and cancellations have been par for the course for months now. Staffing shortages affecting airlines and traffic control personnel have caused ground stops throughout the summer travel season. The FAA said today it has limited ability to direct New York flights north through Canadian airspace because of "staffing issues" in Canada.