Have you ever seen a cloud that looks like a tube of cotton rolling along parallel to the horizon? If you have, you’ve seen the relatively rare roll cloud. This cloud is associated with the gust front of a thunderstorm, or occasionally with a cold front. They are completely detached from the thunderstorm base or other cloud features. Although they appear to “roll” along the horizon, they are not funnel clouds. Still, it doesn’t pay to dismiss them. Roll clouds have been associated with straight line winds in excess of 100 miles per hour.
Mary Knapp, Weather Data Library