A gust front is the leading edge of the cool outflow air pushed out of a thunderstorm by its downdraft. Its passage is usually marked by a sharp wind shift, a temperature drop, a rise in pressure, blowing dust, and often a shelf cloud along the boundary.
How it forms
Precipitation falling through a thunderstorm drags air downward. As drops and small hail evaporate, they cool the air around them further. That cooler, denser air accelerates to the surface as a downdraft. When it hits the ground, it spreads out horizontally in every direction as a shallow pool of cold air, and the leading edge of that pool is the gust front.
Because the outflow is denser than the warm environment ahead of it, it behaves like a small cold front on a very short timescale. Warm, moist inflow air is wedged upward as it is undercut, which is what produces the shelf cloud or arcus formation often draped along the front.
How forecasters and spotters use it
In the field, the shelf cloud and the leading wind shift are the two most reliable visual and sensory cues. A well-developed gust front can look like a low, wedge-shaped cloud running across the sky, with turbulent motion on the underside and smoother rising motion at the leading edge.
On radar the gust front often shows up as a fine line of weak reflectivity moving ahead of the parent storm. Forecasters watch these boundaries because new convection frequently fires where a gust front collides with warm inflow or with another boundary, and because two gust fronts colliding is a common trigger for surprise new storms.
Important limits
The strongest damaging winds are usually right at or just behind the gust front, not deep inside the rain. Assuming the worst has passed once the wind slackens can be a mistake, especially in bow echoes and derecho events where secondary surges arrive after the initial front.
A gust front can also outrun its storm entirely and continue as an outflow boundary for hours, moving through areas with clear skies. If you are in the threatened region, plan to be indoors before the leading edge arrives, not after.