This structure usually marks the location of the thunderstorm updraft. This is particularly true when the rain-free base is observed to the south or southwest of the precipitation shaft. It can often be seen as a clear space of air wrapping around the wall cloud. Scattered large precipitation between the clear slot and wall cloud may show up on radar as a hook echo. Usually occurs on rivers, after flash flooding has occurred on streams and tributaries.
River floods develop and reach their peak more slowly than flash floods.
In many cases, the river flood peak occurs after the rain has ended. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. Roll clouds are not and do not produce tornadoes Scud Clouds Low cloud fragments often seen in association with and behind thunderstorm gust fronts. Scud clouds do not produce severe weather. In some cases, when clouds are attached to the thunderstorm base, they can be mistaken for wall clouds or tornadoes.