A back-sheared anvil is a thunderstorm anvil that spreads against the upper-level wind, extending upshear from the storm's updraft rather than only downwind, and it points to an updraft strong enough to overcome the flow at anvil level. Visually, the storm looks like it is throwing cloud material out both ways at its top, with a distinct edge pushing into the wind.
Why the anvil pushes upshear
In a typical storm, the updraft reaches the tropopause and its ice crystals are carried downstream by the strong winds that live at that altitude. The anvil forms a long plume in the direction of the upper flow. In a very strong updraft, the rising air arrives at the tropopause with enough horizontal spread and momentum that some of it fans out against those upper winds before being turned back downstream.
Because the effect requires the updraft to work against the ambient flow, the size of the back-sheared portion scales with how strong the updraft is relative to the winds at anvil level. A pronounced upshear extension implies the updraft is winning that contest by a comfortable margin.
How spotters read it
From the ground, back-shearing looks like a sharp anvil edge or a curling lip on the side of the storm that faces into the wind aloft. Combined with a persistent overshooting top and a hard, crisp downwind anvil edge, it forms one of the clearest visual verdicts a chaser can make about updraft health.
Time behavior matters. A back-sheared appearance that holds through multiple radar volumes is more informative than a brief flare during a single pulse. If the storm loses the upshear edge and the anvil begins to feather downstream only, that can indicate the updraft is weakening.
Where it misleads
Weak upper-level flow can produce an anvil that appears to spread in every direction without any exceptional updraft, because there is little wind to push it in one direction. Judging back-shearing in a low-shear environment is unreliable and does not carry the same signal as it does under strong upper winds.
The signature also reflects conditions aloft, not what is happening at the surface. A textbook back-sheared anvil can appear before any storm base features have organized, and the base underneath still has to be read on its own for tornado potential.