Undulatus Asperatus was first identified last year by Cloud Appreciation Society founder, Gavin Pretor-Pinney and is currently being researched by Reading University.
The US National Weather Service writes:
“Undulatus asperatus (or alternately, asperatus) is a rare, newly recognized cloud formation . . . The clouds are most closely related to undulatus clouds. Although they appear dark and storm-like, they tend to dissipate without a storm forming. The ominous-looking clouds have been particularly common in the Plains states of the United States, often during the morning or midday hours following convective thunderstorm activity.”
You can read more and see some very good photograps here