Frontier Airlines says 13 of its flights from Denver International Airport were canceled Wednesday because its aircraft were damaged by hail.

All flights were grounded Wednesday afternoon because of a tornado warning and severe thunderstorms that brought hail and strong winds to the area. There were delays of about an hour and a half after operations resumed.

Up to eight tornadoes touched down in the Denver metro area.

At least one tornado touched down in the town of Watkins, southeast of the airport, but no injuries were reported.

Hail the size of ping pong balls sent pedestrians running for cover in Colorado Springs, and cars slid on the hail on Interstate 70 in Aurora.

The hail piled up several inches deep, dented cars and broke windows out of some homes.

Officials used snow plows to clear the hail from the main airport access road.