From AMS Glossary
(Also called streamers.) The trails of precipitation that emerge from the base of generating cells typically observed on time–height displays from vertically pointing radars.
Snow trails are also commonly observed on range–height indicator displays. Snow trails emerge from a layer of convective instability that often exists in the middle or upper troposphere in widespread storms. Small convective cells developing within this layer produce the ice crystals that then fall to lower altitudes. The base of the convectively unstable layer is called the snow- generating level. The shape and vertical extent of the streamers depend on the vertical profiles of wind and relative humidity in the layer through which the precipitation falls.