From Glossary of Meteorology
A weak radar echo in the form of a tapered radial extension beyond the far edge of a severe thunderstorm, associated with the presence of large hydrometeors, especially hail, in the storm.
The phenomenon is explained by what is referred to as "three-body scattering." Some of the radar energy is scattered from the hail region to the ground, reflected back to the hail region, and then scattered a third time, eventually returning to the radar as a weak signal. From the perspective of the radar, the signal travels out and back with a timing indicating that the scattering region is beyond the back edge of the storm. Flare echoes are observed more commonly with radars operating at shorter wavelengths, evidently because of deviations from Rayleigh scattering.