(AP) – The unofficial start of summer is offering a troubling look at what lies ahead for travelers during the peak vacation season.

U.S. airlines canceled more than 2,800 flights over a five-day stretch including the Memorial Day weekend, as bad weather, crews calling in sick, and other factors snarled their operations.

Delta Air Lines, which is usually among the top performers, had the worst record among major carriers with more than 800 canceled flights from Thursday through Monday.

The Transportation Security Administration reported screening more than 11 million people at airport checkpoints during that timeframe.