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An outflow boundary is a sudden burst of air from a thunderstorm. These burst forth from the cloud, often sweeping out in a wide arc. An outflow boundary generally occurs near the end of a thunderstorm's life, when the heavy rain building up becomes too much for the updraft in the storm. Most of the storm quickly converts to rapidly sinking air as the convective column collapses. This often results in an extremely heavy burst of rain and a strong gust of cold air (the cold blast of air you always feel right before rain begins.)

Near the storm, outflow boundaries can be rather severe, kicking up dust and making a cloud shelf as they pass across areas beyond the storm. In these cases they are sometimes known as a gust front. These gust fronts may move hundreds of miles from the original storm, and even can act like a cold front, causing new storms to form.

Although in normal, isolated summer storms an outflow boundary signals a storm nearing the end of its life, this is not always the case. In the case of mesocyclones or squall lines, the storm may produce such formations and remain extremely powerful. Also, the start of an outflow boundary can sometimes be in the severe form of a microburst, which can cause localized tornado-like damage, and can cause low-flying planes to crash.

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