Now questions abound: What is it exactly? And what are the chances you'll get to see one in person?

It's called a "roll cloud," which is a type of arcus cloud—low, horizontal formations typically associated with thunderstorms. Depending on the conditions, a roll cloud "can last for several hours and extend for several hundred miles," said Stephen Corfidi, a meteorologist at the National Oceanic and Atmospheric Administration (NOAA).

The rolling motion is the result of winds changing speed and/or direction at the inversion—when the air temperature reverses from its usual state, resulting in warm air on top of cool air—along which the weather disturbance is traveling. "The 'shear' across the inversion sets up a rolling motion much like that of