A storm interaction is any change in a storm's structure or behavior that comes from another storm nearby, its cold outflow, or a shared inflow layer. Interactions are more general than mergers and include effects that happen well before, or entirely without, two cells joining.
How storms influence each other
The most common interaction is through outflow. An outflow boundary from an upstream storm can lift the inflow air feeding a downstream storm, sometimes locally enhancing low-level rotation, sometimes cutting off the storm's warm inflow entirely. Storms in close proximity also compete for the same low-level moisture; if one storm is pulling on the same reservoir as another, both may weaken.
Baroclinic boundaries created by upstream cold pools have been implicated in tornado intensification. When a supercell crosses an outflow boundary, the enhanced horizontal vorticity along that boundary can be ingested and tilted into the updraft, adding rotation at low levels.
How forecasters read interactions
Radar loops and mesoanalysis together tell most of the story. Warning meteorologists watch for outflow boundaries painted by fine lines or debris on reflectivity, then look at where those boundaries are drifting relative to nearby storms. Boundary intersections with a supercell are treated as short windows of elevated tornado potential.
A storm that has just crossed onto the cool side of an outflow is treated with suspicion, because it may lose its inflow within one to two volume scans. Chase teams in the field also use surface temperature and dew point drops to confirm what radar suggests about which side of a boundary the storm is on.
Important limits
Storm interactions are hard to predict in specifics. The same outflow boundary can strengthen one supercell and destroy the next one, depending on the storm's relative motion, the depth of the cold air, and the moisture on either side. Interactions are best treated as reasons to watch a storm more carefully rather than as signals of a specific outcome.
When the field team or the radar shows an intersection developing, the message is that the storm may change quickly, not that any particular change is coming.