More than 600 flights were canceled and thousands more were delayed in the United States on Saturday, according to the flight tracking website FlightAware. Saturday marks the third day of major flight cancellations after thunderstorms pounded major airports on the East Coast on Thursday, making for one of the worst days for flight cancellations of the past six weeks. This summer travel season has been plagued with flight cancellations and delays as airlines contend with staffing shortages, severe weather and air traffic control delays. As of Saturday evening, at least 636 flights into, within or out of the United States were canceled, and an additional 5,921 domestic flights were delayed, according to FlightAware. Newark Liberty International and Chicago Midway International Airport topped the list of US airports with the highest number of canceled flights, the flight tracking website reported. American Airlines canceled 4% and delayed 24% of its Saturday flights, according to the site. Meanwhile, United had 4% of its flights canceled and 23% delayed, and Delta had 2% canceled and 22% of flights delayed, FlightAware reported. About 41% of JetBlue’s flights and 36% of Southwest’s flights were also delayed on Saturday, according to the website. “We continue working through a variety of weather related challenges across the country this weekend,” Southwest Airlines told CNN in a statement on Saturday. “Our teams are working to get our customers to their destinations safely and as quickly as possible.” CNN has reached out to the other domestic carriers for comment about Saturday’s delays and cancellations. Amid the flurry of flight cancellations and a flood of complaints from passengers, the US Department of Transportation on Wednesday proposed expanding the circumstances when airline passengers can get refunds. The department said it was inundated with complaints from airlines passengers seeking refunds since the outset of the Covid-19 pandemic. The proposed refund rules come after a private June meeting that Pete Buttigieg held with airline CEOs in which he told them to fix the summer flight schedules that have been bedeviled with thousands of cancellations.