A hook echo is a pendant- or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flows into a mesocyclone resulting in a curved feature of reflectivity. The echo is produced by rain, hail, or even debris being wrapped around the supercell. It is one of the classic hallmarks of tornado-producing supercells. The National Weather Service may consider the presence of a hook echo as sufficient to justify issuing a tornado warning.
http://en.wikipedia.org/wiki/Hook_echo
http://en.wikipedia.org/wiki/Hook_echo