CWG posted about it.
This cloud type is known as a roll cloud or arcus cloud.
Resembling a giant dough roller, a roll cloud is completely detached from its parent shower or thunderstorm.
These clouds are relatively rare, less common than its cousin the shelf cloud, which often can be found along the leading edge of thunderstorms, forming along their gust fronts. Roll clouds seldom produce violent winds, whereas shelf clouds are sometimes a harbinger of severe weather.