squall line

A narrow, elongated band along which a group of thunderstorms is arranged. The wind shifts abruptly in direction and increases in intensity as this non-frontal storm line passes, producing the squall referred to in its name. The most severe squall lines are preceded by an ominous-looking cloud wall that takes the shape of a roll (arcus). Besides the violent wind gusts they generate, the storms leave brief but intense downpours, often accompanied by hail.