The awesomeness of God ...
We believe it is a roll cloud — a low, horizontal, tube-shaped, and relatively rare type of arcus cloud.
Roll clouds usually appear to be "rolling" about a horizontal axis.
The
roll cloud is not attached to its parent thunderstorm. It's associated with a gust front or outflow boundary out ahead of the thunderstorm.
These clouds form when warm, moist air is lifted by rain-cooled air moving underneath it caused by the storm's downdraft or gust front. When the warm, moist air is lifted it condenses and forms the cloud.
I saw clouds like this once, while driving. Came around a bend and slammed on my brakes (like everyone else) because it legitimately looked like a huge wave was about to crash down on us. It was incredible. Really beautiful after I understood I wasn’t about to die," one user commented.
See it in motion here: