The rear-flank downdraft, or RFD, is a region of sinking, often relatively dry air that develops on the back side of a supercell's rotating updraft. It wraps cyclonically around the low-level mesocyclone and is closely linked to tornado formation.
How the RFD forms
Two families of processes produce an RFD. Thermodynamic forcing includes evaporation, melting, and precipitation loading along the trailing side of the storm, all of which cool air and drive it downward. Dynamic forcing comes from the rotating updraft acting as an obstacle to mid-level, storm-relative flow. Pressure perturbations develop on the upshear side of the rotating updraft and force air to descend even without strong negative buoyancy.
The relative contribution of these two families changes from case to case and, in a single storm, from one surge to the next. A dynamically dominant RFD can be relatively warm and dry, while an RFD dominated by heavy precipitation and evaporation is more likely to be cool and stable.
Structure at the surface
When the RFD reaches the ground, its leading edge wraps around the low-level mesocyclone from the west and south, producing a curved gust front that can be seen on radar as a hook echo. On the visible cloud base, spotters see it as the clear slot cutting into the rear of the updraft, sometimes producing a horseshoe-shaped opening around the wall cloud.
Repeated RFD surges can leave successive gust-front arcs and are the mechanism behind cyclic tornadogenesis, in which one tornado dissipates and a new mesocyclone develops just to the east of the old one.
How forecasters use it
Radar signatures that suggest an RFD surge include a tightening hook, an RFD notch on the reflectivity display, and a strengthening low-level velocity couplet immediately adjacent to the hook. When these features contract and low-level rotation intensifies over successive volume scans, tornado warning confidence increases.
Field observations that support the same picture include a warm, dry gust front pushing east of the wall cloud, a visible clear slot wrapping around the base, and rapid tightening of a rotating lowering. Warning meteorologists integrate these observations with the environmental context to judge whether the storm is likely near tornadogenesis.
The thermodynamic distinction
Field campaigns beginning with the original VORTEX project and continued through VORTEX2 have documented that tornadic supercells tend to have RFDs with smaller near-surface temperature deficits and higher equivalent potential temperature than nontornadic ones. A warmer, more buoyant RFD is more able to sustain the strong upward motion at the surface that stretches near-ground vorticity into a tornado.
This is a statistical tendency observed across many cases, not a rule that fires storm by storm. Individual storms can violate it, and warm RFDs alone do not guarantee tornadoes. It does explain, however, why the visible clear slot on the rear of a supercell is one signal among several, not the deciding one.
What the RFD does not prove
A visible RFD surge does not confirm a tornado is on the ground. Many supercells produce multiple RFD surges without ever spinning up a tornado at the surface, and the storm's behavior depends on low-level shear, boundary-layer moisture, and the storm-relative inflow available to the updraft.
Chasers and spotters should treat an RFD wrapping around a mesocyclone as a call to be positioned safely, not as permission to close in. If you are in the threatened region, have a plan to take shelter if a warned storm approaches you.