Storm outflow is the pool of relatively cool air that spreads outward along the ground after descending in a thunderstorm downdraft. It behaves as a shallow density current, with a distinct leading edge called the gust front.
How the cold pool works
When a downdraft reaches the surface, it fans out because it cannot go any further down. The rain-cooled air is denser than the surrounding boundary layer, so it hugs the ground and pushes outward against warmer air. The horizontal pressure difference at the leading edge drives the gust front forward, much like the leading edge of a small dam release.
Depth and temperature deficit set how fast the outflow moves and how effectively it can lift warm inflow at its leading edge. Deep, cold pools race outward. Shallow, marginal pools stall and may be shredded by the ambient wind.
How forecasters read outflow
On radar, the outflow appears as a thin fine line of enhanced return where insects, dust, and small returns concentrate at the convergence zone. On surface observations, outflow arrival shows up as a sharp wind shift, a temperature drop, a pressure rise, and a dew point change whose sign depends on how dry the descending air was.
Balance between the cold pool and low-level shear controls line organization. When the two are in rough balance, updrafts along the gust front remain upright and long-lived. When the cold pool wins, storms tilt over their own outflow and weaken. When shear wins, the leading edge slows and new storms can back-build.
Where outflow misleads
Outflow is not always a signal of a dying storm. A surging cold pool can regenerate new updrafts on its leading edge, which is the basic building block of a multicell cluster and of many MCS events. Reading outflow as end-of-storm without checking the environment ahead of the boundary is a common mistake.
Outflow boundaries also persist long after their parent storm dies. A leftover boundary can enhance low-level shear and convergence for hours, so the atmosphere downstream of yesterday's storms is not the same atmosphere the model sounding depicts.