A hook-shaped radar echo often linked with tornadic supercells is called a hook echo.
It appears when precipitation, hail, or debris wraps around the rotating updraft and mesocyclone, creating a curved feature in the storm’s lower reflectivity pattern. The hook commonly forms near the interaction between the main updraft and the rear-flank downdraft.
A hook echo is an important warning clue, but it does not prove that a tornado is on the ground. It primarily signals a strong mesocyclone, and some hook echoes never produce tornadoes. Conversely, high-precipitation supercells can obscure the classic hook, so forecasters also examine Doppler velocity couplets and, where available, debris signatures.
The first documented association between a hook echo and a confirmed tornado came near Champaign-Urbana, Illinois, on April 9, 1953. Donald Staggs noticed the unusual shape on experimental radar.