The sky in Sioux Falls, South Dakota, darkened and turned an eerie shade of green on Tuesday as a derecho swept through the region.
A derecho is a cluster of thunderstorms and meteorologists will only classify a windstorm as such if the wind damage will cover a distance of at least 400 miles.
Onlookers and weather experts shared images of the phenomenon before the storm hit.
According to local Fox Weather meteorologist Heather Brinkmann, the green color is created from light refraction within the thunderstorms and indicates that the storm will be capable of producing large hailstones.
It takes a “tremendous amount of water content within the cloud to achieve this color, which usually means a substantial amount of ice (large hail) has to be present,” National Weather Service meteorologist Cory Martin shared in a graphic.
Indeed, there were later reports of large golf ball-sized hail in parts of the region.