Flumen are bands of low clouds associated with a severe, supercell, convective storm Cumulonimbus (Cb), that are arranged parallel to the low-level winds and moving into or towards the supercell.
These accessory clouds form on an inflow band into a supercell storm along the pseudo-warm front. The cloud elements move towards the updraft and into the supercell with the base of the flumen being at about the same height as the updraft base. Note that flumen are not attached to the murus wall cloud and the cloud base is higher than that of the wall cloud.
Flumen are often mistaken for tornadoes but they can be easily distinguished as a flumen will not be rotating.
One particular type of inflow band cloud is the so-called 'Beaver's tail'. This is distinguished by a relatively broad, flat appearance suggestive of a beaver's tail.
Flumen cloud at the base of a severe cumulonimbus cell. Source: International Cloud Atlas