RFD is the acronym for rear-flank downdraft in a supercell thunderstorm.
The RFD is a region of descending air that wraps around the back side of a mesocyclone. It can contain relatively dry air and precipitation, and its interaction with the storm’s rotating updraft helps produce the hook-shaped radar pattern known as a hook echo.
Researchers have proposed several ways the RFD can contribute to tornadogenesis. It may transport vorticity toward the surface, enhance convergence beneath the updraft, and help tighten existing rotation. Its exact role varies between storms, and tornadoes can occasionally form without the classic RFD structure.
The RFD is often contrasted with the forward-flank downdraft, or FFD. The FFD is usually associated with the main precipitation core and tends to be cooler and wetter, while the RFD commonly contains drier descending air. Neither acronym should be confused with MCS, QLCS, or CAPE, which describe different storm structures or atmospheric properties.