An outflow boundary is the leading edge of rain-cooled air spreading away from a thunderstorm, separating the cooler storm outflow from the warmer air the storm was ingesting. It is a mesoscale boundary produced entirely by convection, and it behaves like a shallow cold front once the parent storm has weakened or moved away.
How it forms
Rain falling through a thunderstorm cools the surrounding air by evaporation and drags it downward. When the cold air reaches the ground it cannot go further and spreads outward in all directions along the surface, pushing the warmer environmental air upward at its leading edge.
When the parent storm is still producing, the leading edge of that spreading cold pool is the gust front. As the storm decays or moves on, the cool pool detaches, keeps spreading, and becomes an outflow boundary that can persist for many hours and travel hundreds of kilometers from where it originated.
How forecasters use it
Outflow boundaries show up on radar reflectivity as thin fine lines and on velocity as narrow zones of shear. Surface observations along the line show a wind shift, a temperature drop, and often a rise in dewpoint from evaporated rainfall. Together those signatures let a forecaster track a boundary long after the storm is gone.
The boundary matters because it can trigger new storms where it intersects a dryline, a warm front, or another outflow, and because it enhances low-level shear locally. A supercell that crosses an outflow boundary from the warm side can sample stronger streamwise vorticity, which sometimes supports rapid low-level mesocyclone intensification.
Important limits
Outflow can also cut off a storm's inflow. A cell that becomes undercut by its own cold pool loses access to the warm, moist air it needs and shifts into a linear or messy mode. On radar this looks like the updraft base pulling apart from the reflectivity core, with new cells firing along the leading edge instead of in the parent.
Outflow boundaries are hard to represent in numerical models, especially convection-allowing runs that produce their own storms with their own cold pool errors. A model that places a boundary in the wrong county at initiation time can produce a very different afternoon than what unfolds. We'll be watching for changes.