This turquoise tinge has been commonly attributed to hail , but its actual cause is unknown. A horizontal, dark cumulonimbus base that has no visible precipitation beneath it. 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 . 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 .