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. As of June, 2009 the Royal Meteorological Society is gathering evidence of the type of weather patterns in which undulus asperatus clouds appear, so as to study how they form and decide whether they are distinct from other undulatus clouds.
One more pic after the jump: