An arcus cloud is a rather long bar or shelf which extends outward from the front side of the base of a well-organized thunderstorm. It's common name is a shelf cloud and it is caused by the condensation of water vapor in warm and humid air being forced upward by the cooler gust front along the leading edge of a well-organized thunderstorm. When observed from a distance, it's appearance can resemble a flying saucer.
A shelf cloud is not a wall cloud and is not necessarily associated with tornadoes. However, shelf clouds are associated with the gusty winds along the leading edge of a thunderstorm. Due to the wide open spaces here in the Northern Plains, shelf clouds often look quite dramatic, but the amount of drama is not necessarily related to the speed of the wind. Some shelf clouds produce just a faint puff of air while others generate truly severe thunderstorm winds.