The forward-flank downdraft, or FFD, is the region of cooled, sinking air associated with the main precipitation shaft of a supercell, generally located downshear of the rotating updraft.
How it forms
As precipitation falls out of the updraft, it is carried downshear by upper-level flow and drops through the mid- and low-levels of the storm. Water loading, melting of graupel and hail, and evaporation of raindrops all cool the air along the way, producing negative buoyancy. That negatively buoyant air sinks and spreads out across the ground, forming a rain-cooled pool northeast of the mesocyclone in a classic northern-hemisphere supercell.
The FFD is a broader, longer-lasting downdraft than the rear-flank downdraft. It is fed continuously by the fallout from the storm's main precipitation core rather than by a compact surge on the storm's back side.
Why forecasters care about it
The temperature contrast between the FFD outflow and the warm inflow air produces baroclinic horizontal vorticity along the forward-flank boundary. When storm-relative inflow sweeps that vorticity into the updraft base, it can be tilted and stretched into vertical rotation, contributing to the low-level mesocyclone.
On radar, the FFD hosts the storm's core reflectivity. Dual-polarization signatures often show a large-drop, hail-mixed core embedded in the FFD, and differential reflectivity arcs mark the size-sorted leading edge of the precipitation. Together these help forecasters locate the boundary between the FFD and the inflow region.
Where it misleads
The FFD is not always cool. Research has shown that the thermodynamic character of the forward flank varies from case to case, and near-surface temperature deficits within the FFD can be modest in some tornadic supercells. Assuming a strongly cold FFD is baked into every storm can lead to wrong inferences about how much low-level buoyancy actually reaches the updraft.
The FFD is also not a safe area to observe from. Storm-motion vectors can shift, hook echoes wrap through the forward flank in high-precipitation supercells, and tornadoes have occurred in the FFD region during occlusions and cyclic tornadogenesis. It is a hazardous side of the storm, not a viewing platform.