Todd Heitkamp, a National Weather Service meteorologist, said the line of storms appeared to coalesce into a derecho around Sioux Falls.

The last time a derecho hit eastern South Dakota, the sky above Sioux Falls turned green, prompting Reddit users to create images of everything from Cthulhu to the Stay-Puft Marshmallow Man rampaging through the city. Here's what happened the last few times derechos have occurred in South Dakota.

A dangerous derecho is expected to form in parts of the Northern Plains and Upper Midwest on Monday, with wind gusts over 75 mph likely. A moderate threat for severe storms is in place for parts of South Dakota and Minnesota on Monday afternoon into the evening due to the threat of a derecho, a wind storm that can cause significant damage.