Hundreds of flights have been delayed at Denver International Airport, with authorities noting severe weather and thunderstorms in the region.

Meanwhile, the National Weather Service issued a tornado watch for Denver, Fort Collins and several other counties.

The weather service also said the threat of very large hail — more than 3 inches — is increasing on Monday afternoon, especially for areas near I-70 from just east of downtown Denver to the Kansas border.

“Take the necessary safety precautions if a warning is issued!” the weather service warned.

Flightaware.com said roughly 350 flights have been delayed, though there had been only two flight cancellations. It’s not immediately clear what caused the flight cancellations.

In a status update, the Federal Aviation Authority said a traffic management program has been implemented at DIA for arriving fights due to “weather/thunderstorms.”