Rear flank downdraft
From Wikipedia, the free encyclopedia

The rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of decending air are thought to be essential in the production of many supercellular tornadoes. Large hail within the rear flank downdraft often shows up brightly as a hook on weather radar images, producing the characteristic hook echo, which often indicates the presence of a tornado.
[edit] See also
[edit] References
A Comprehensive Glossary of Weather Terms for Storm Spotters