A stationary front is a boundary between warm and cold air masses that is moving very slowly or not at all, because neither air mass is advancing strongly against the other. On a surface map it is drawn with alternating blue triangles and red bumps pointing in opposite directions, one set into the warm air and one set into the cold.
How it forms
A cold or warm front can become stationary when the pressure pattern steering it weakens or when it runs into terrain or a competing air mass. The temperature contrast across the boundary remains, but the horizontal wind field no longer has a net component pushing one air mass across the other.
Along the boundary, warm air on the south side still tries to override the cold air on the north side. That gentle upglide, combined with any low-level moisture convergence, produces stratiform rain, embedded thunderstorms, and often a persistent cloud shield.
How forecasters use it
Because the boundary does not move, storms that fire on it tend to move parallel to the line. That is the classic setup for training storms: cell after cell tracks over the same counties for hours, producing rainfall totals well above what any single storm would drop.
The forecast question is usually when the front will start to move again. Watching upstream shortwaves, jet streaks, and low-level thermal advection tells a forecaster whether the pattern will finally push the boundary north as a warm front or south as a cold front.
Important limits
Stationary fronts are notoriously hard to place precisely. A slow-moving boundary can wobble tens of kilometers in either direction under diurnal heating and mesoscale outflows, and small position errors change which corridor sees the heaviest rain. Radar, satellite, and mesonet trends usually place the boundary better than any single forecast product.