A hook echo on the radar image of the May 3, 1999 supercell thunderstorm that produced an F5 tornado in Moore, Oklahoma.

A hook echo is a hook shape seen on a weather radar of some supercell thunderstorms. It is produced by rain, hail, or even debris wrapping around the thunderstorm. A hook echo is a sign that a tornado has formed or is forming. If a hook echo is seen on radar, the National Weather Service may issue a tornado warning. view more...