Little Chute, Wis.
As a storm cell swept across eastern Wisconsin on June 13, 2004, it was led by a haunting shelf cloud, seen here over the town of Little Chute.
The storm made for some stunning images from Greenville to Green Bay, but luckily it was less severe than the photos suggest. "Although its appearance is threatening, and almost always precedes gusty winds, the [shelf] cloud is not necessarily a precursor to severe weather," the National Weather Service explains. Still, shelf clouds can generate dangerous straight-line winds, including the rare "derecho" and "gustnado," and shouldn't be taken lightly.