Thousands of flights across the U.S. were canceled or delayed Monday as powerful storms swept across the eastern half of the country and a partial government shutdown affecting airport security screeners dragged into a second month.

The disruptions come at an already challenging time for air travel, in part because the shutdown that began Feb. 14 has strained staffing at some security checkpoints. At the same time, airports are crowded with spring break travelers and fans heading to March Madness games, the annual NCAA men’s and women’s college basketball tournaments.

Flight delays and cancellations piled up Monday at some of the nation’s largest airports, including those in New York, Chicago and Atlanta. The storm system that dropped snow by the foot in the Midwest was barreling toward the East Coast with dangerously high winds and the potential for “producing strong and long track tornadoes,” the National Weather Service warned Monday.

More than 3,600 flights scheduled to fly into, out of or within the U.S. on Monday have been called off, according to flight-tracking site FlightAware. Roughly 6,800 other U.S. flights were delayed.

The nationwide cancellations included nearly 500 in and out of Chicago O’Hare International, more than 300 at Atlanta’s Hartsfield-Jackson International and over 230 at John F. Kennedy International Airport, according to FlightAware.

Earlier Monday, citing severe weather, the Federal Aviation Administration ordered ground stops at Hartsfield-Jackson and Charlotte Douglas International Airport, along with ground delays at JFK and Newark Liberty International Airport.

Danielle Cash found herself stranded in St. Louis on Sunday while trying to get home to Tampa, Florida, after a weekend girls’ trip to Las Vegas. Now she’s spending several hundred dollars more than planned on a hotel room in a snowy city she wasn’t dressed for.

“It was 80 degrees in Tampa when I left and then going to Vegas,” she said. “And it was 90 degrees in the desert.”

Cash said she’s now booked on a flight that will take her to Tennessee before finally returning to Tampa by Tuesday afternoon.

The storms are also unfolding just as airport security screeners missed their first full paycheck over the weekend. The current partial government shutdown affects only the Department of Homeland Security, which includes the Transportation Security Administration. It is the third shutdown in less than a year to leave TSA workers temporarily without pay.