Sometimes the boundary between two air masses stalls and shows little movement for an extended period. That boundary is a stationary front. Neither air mass is advancing strongly against the other.

Weather along a stationary front can include persistent clouds and light to moderate precipitation, especially if moist air is forced to rise over the denser air. The front may eventually begin to move again as a cold or warm front once one air mass gains strength.

Stationary fronts often appear on maps as alternating red semicircles and blue triangles on opposite sides of the line. They can linger for days, producing prolonged cloudy and damp conditions over the same region. Flooding becomes a risk if rainfall continues over the same area.

Forecasters watch for any shift that indicates the front is about to begin moving.