from the facebook page
This is an utterly fantastic image, taken by Jeremy Holmes, of a Nebraskan supercell.
A supercell (also known as a rotating thunderstorm) is a type of thunderstorm that forms in the presence of rotating updraft. Overall, supercells are less common than classic run-of-the-mill thunderstorms, which is fortunate since supercells have the potential of being very severe. They are most commonly identified by their ominous rotating structure and usually occur in arid climates (such as the Great Plains of the United States), but supercells can form anywhere in the world under the right conditions.
Sources and further reading: http://www.fromquarkstoquasars.com/nebraskan-supercell