When the storms rumbled through this weekend, they produced some ominous looking clouds.
Viewers posted several pictures of scary clouds to the KSLA StormTracker 12 Facebook. A few even commented that they have never seen a cloud formation like this or did not know what type of cloud it was.
Well, what the viewers were seeing was either a shelf or a roll cloud. Both clouds are a type of arcus cloud, a low, horizontal cloud.
The main difference between a shelf cloud and a roll cloud is a shelf cloud forms on the leading edge of a thunderstorm. 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 the storms downdraft or gust front. When the warm, moist air is lifted it condenses and forms the cloud.
Typically, when a shelf cloud or roll cloud passes one point the wind picks up and changes direction. If a shelf cloud passes, then heavy rain and hail could follow as well.