A flanking line is an organized band of growing cumulus and towering cumulus that extends outward from the main updraft tower of a supercell or strong multicell storm, usually on the inflow side. Cells in the line step upward toward the parent storm and feed the main updraft.
How it forms
As a mature storm produces outflow along its downshear side, cold air spreads out along the surface and meets the warm, moist environmental air moving into the storm. Along that convergence zone, air is forced up in a line of new cumulus. The cumulus nearest the parent updraft is the tallest, and the towers become progressively shorter along the line away from the storm.
This stair-step appearance is the flanking line's signature. Each new tower is a fresh convective element that either merges into the main updraft or takes a turn as the dominant updraft as the storm cycles.
How forecasters and spotters read it
A well-organized flanking line is a sign of a healthy storm with a reliable inflow supply. Because the line marks the boundary between environmental inflow and storm-generated outflow, it also shows where the next round of updraft development is likely to occur.
In supercell reporting, a flanking line that continues to build cleanly usually indicates the storm will persist. A flanking line that goes ragged, shrinks, or is overrun by outflow suggests the storm is losing its balance with its environment.
Important limits
Flanking lines occur in many kinds of organized deep convection, not just tornadic supercells, and they say nothing directly about rotation. The main storm still has to be evaluated on its own radar and visual signatures.
The line can also become confused with a separate line of storms firing on a nearby boundary. If cells along the line are not stepping up toward the main storm and merging, they may be their own independent convection, not a true flanking line.