Shelf cloud photographed in Onalaska, Wis., May 16. (Kyle Nyman via Twitter)

As a vigorous complex of thunderstorms barreled southeast through central and southern Wisconsin Thursday morning, it was fronted by a menacing, wicked-looking cloud formation.

The dark, protruding, multilayered clouds were known as shelf clouds or “shelfies,” common in thunderstorms.

Some of the best examples came from Madison, where the storms swept through between 8:30 and 9 a.m. local time. The storms in the state’s capital had more bark than bite — producing just brief downpours and wind gusts to around 30 mph.

Shelf clouds form from cold air or outflow racing out ahead of the storm, which sinks and clashes with warm, moist air feeding into it. The humid air rises quickly through this cold layer and condenses into a horizontal cloud resembling a shelf. Sometimes, this happens in multiple layers resulting in a stunning shelf deck resembling a massive flying saucer or “mothership.”

As a shelf cloud arrives, there is usually a sudden burst of heavy rain and strong winds. Then the intensity of the storm tends to ease.

The thunderstorm complex formed ahead of a cold front pushing southward through the Great Lakes toward northern Illinois, Indiana and Ohio. In this vicinity, the National Weather Service has outlined a slight risk zone for severe thunderstorms through Thursday evening.

See more stunning views of the shelf clouds associated with these storms below...


Shelf cloud seen from roof of Atmospheric and Oceanic Sciences building at the University of Wisconsin-Madison. (UW-Madison SSEC/AOS)

More shelf clouds:

Shelf clouds sweep over Washington region

Shelf clouds over the Chesapeake Bay

The dramatic shelf clouds that devoured the Wisconsin skies

A wicked-looking shelf cloud rolls through Kentucky and Ohio (Photos)

Time lapse: Shelf cloud, hail sweep through Sydney

Photos: Ominous shelf clouds roll through Nebraska and Kentucky

Towering ‘mothership’ storm mesmerizes and menaces Washington region