What hook-shaped radar signature is a classic sign of a tornadic supercell? It is called a hook echo. On weather radar, a hook echo appears as a curved or pendant-shaped area of reflectivity near the lower part of some supercell thunderstorms.
The shape forms as rain, hail, or debris wraps around the storm’s rotating updraft, or mesocyclone. Air and precipitation flow into the circulation, producing a curved appendage on the storm’s radar image. Meteorologists watch the hook alongside other evidence, including velocity signatures that reveal rotation toward and away from the radar.
A hook echo is an important warning clue, but it is not a photograph of a tornado and does not prove that a tornado is on the ground. It more directly indicates a storm-scale circulation capable of producing one. The National Weather Service may use a hook echo together with a tornado vortex signature and other observations when deciding whether to issue a warning.
The feature can be difficult to see. High-precipitation supercells may hide the classic hook in heavy rain, and radar resolution can obscure small-scale structures. A bow echo is a different radar pattern associated mainly with damaging straight-line winds, while a gust front marks the leading edge of thunderstorm outflow.