A storm split is the division of a single strong thunderstorm into two members that then follow diverging paths, with one moving to the right of the mean wind and the other to the left. The two members typically develop opposite rotation, with the right-moving cell rotating cyclonically and the left-moving cell rotating anticyclonically.
The physics behind a split
In an environment with meaningful vertical wind shear, the horizontal vorticity in the shear layer is tilted upward by the updraft. This produces a couplet of vertical rotation, cyclonic on one flank of the updraft and anticyclonic on the other. Each rotating lobe generates its own dynamic low pressure at mid-levels, which draws additional upward motion into its flank of the storm.
Precipitation loading and evaporative cooling then produce a downdraft in the middle of the parent storm. That downdraft cuts through the original updraft and separates the two rotating lobes. What was one storm becomes two, each anchored on its own rotating updraft.
How the wind profile chooses a winner
The environment decides how symmetric the split is. On a nearly straight hodograph, the two members can be roughly equal in strength, with an anticyclonic left-mover that behaves much like the cyclonic right-mover. On a curved hodograph, streamwise vorticity is preferentially fed into the cyclonic member, which usually emerges as the dominant storm while the left-mover weakens.
How forecasters read a split
On radar, a splitting storm typically shows a widening reflectivity core that develops two distinct centers and then two hook-like signatures moving apart. Once the members separate cleanly, they take on different motions and hazards. In the Plains and Southwest, the right-mover is usually the main tornado and large-hail concern; the left-mover, when it survives, is a large-hail and damaging-wind concern.
What a split does not prove
Not every splitting storm produces two supercells, and not every supercell had to split to organize. Splits can also be missed on radar when a downdraft is offset from the geometric center of the storm, so a member may be lost in the imagery. The presence of a split is a useful sign of vigorous rotation and stiff shear, but it does not settle whether either member will be tornadic.