Forecasting
Outflow Boundary Explained: Leftover Storm Air That Can Trigger New Storms
How thunderstorm outflow boundaries form, why they appear on radar, and how they can trigger new storms or change storm direction.
Quick answer: An outflow boundary is a mini cold front created by rain-cooled air from thunderstorms spreading outward.
How it forms
Storm downdrafts spread cool air across the surface.
The boundary between that cool air and surrounding warm air can persist after the storm weakens.
Why forecasters watch it
New storms can form along outflow boundaries if warm moist air is lifted.
Boundaries can also change storm motion or increase low-level spin locally.
Radar clues
Outflow boundaries can appear as fine lines on radar even without rain.
They are easier to notice in loops than on one frame.