A beaver tail is a specific type of inflow band on a supercell, a low, broad, flat cloud band that extends from the precipitation region back toward the updraft base along the boundary between the inflow air and the storm's forward-flank downdraft. Its smooth, flat, tapering shape is the source of the name.
How it forms
A mature supercell produces a shallow pool of rain-cooled air on its forward flank, where precipitation is falling and evaporating. That cool air spreads outward at the surface and forms a boundary with the warm, moist inflow air being drawn toward the updraft. The boundary behaves like a shallow warm front for the storm, which is why it is often called the pseudo-warm front.
Warm inflow air rides up over the shallow cool air along that boundary. Where the lifted inflow reaches its condensation level, a broad, flat cloud band forms and extends from the precipitation region back toward the updraft base. That band is the beaver tail.
How spotters read it
When you can see a beaver tail, you have a useful anchor for the rest of the storm. The precipitation core sits on the far end of the band, the updraft base and any wall cloud sit at the near end, and the boundary itself gives away the location of the forward-flank cool air. This makes it easier to position for a safe view of the updraft.
A prominent beaver tail also indicates that inflow into the storm is strong and organized, which is one ingredient of a mature supercell. It should be read alongside the base under the updraft, not in place of it.
What it does not indicate
A beaver tail is not a tornado precursor by itself. It is the inflow ramp on the storm's cool boundary. Tornado potential still depends on organized, persistent rotation under the updraft base, and the beaver tail neither confirms nor rules out that rotation.
Do not confuse a beaver tail with a tail cloud. A tail cloud is a smaller feature attached to a wall cloud and extends toward the precipitation area with visible upward and inward motion. The beaver tail is broader, flatter, and connects the forward flank to the updraft along the pseudo-warm-front boundary, not the wall cloud.