What are outflow boundaries?

Weather Knowledge

LITTLE ROCK, Ark. – Thunderstorm structure is most easily seen during the summertime in Arkansas when isolated storm development occurs. One feature of thunderstorms that meteorologists will mention quite often is an outflow boundary.

An outflow boundary is created when a thunderstorm produces a downdraft of air that crashes into the ground and spreads out, away from the storm. The downdraft consists of rain-cooled air falling from a storm, since it is more dense than the surrounding air.

As the colder air sinks and spreads, it causes warm and moist air around the storm to rise and condense. This process will help to further spark development of a new storm.

New thunderstorms can also be created where two or more boundaries intersect, forcing air upwards rapidly to condense and bubble up a new rain shower or storm.

Outflow boundaries can be seen in a couple different ways and felt.

When an outflow boundary is produced from a dissipating storm, it will cause winds to shift and the temperature of nearby locations to drop as the colder air flows outward.

Some outflow boundaries can also appear on radar or satellite imagery. On reflectivity mode of radar, they are seen as fine green lines protruding from a thunderstorm. Using satellite imagery, they may be located as arcs of low clouds.

In fact, if you can see it on radar or satellite, there’s a good chance you can also locate it with your naked eye looking at the sky!

Because an outflow of cold, dry air is causing warm and moist air to rise and condense away from the storm, shelf and/or roll clouds may sometimes form along the leading edge of the boundary. If you see these clouds, you can expect some gusty winds and a temperature drop coming your way.

Sometimes you may hear outflow boundaries referred to as “gust fronts.” They act like a very small-scale cold front bring stronger winds and cooler air.

Strong winds associated with the boundary could create storm damage, knocking down trees or power lines or resulting in typically minor structural damage. They also make it difficult for aircraft to takeoff or land as winds shift and increase along the the boundary line.

Sometimes, insects and other small wildlife can be forced to fly in the direction of the boundary as the winds sweep by.

Outflow boundaries can travel hundreds of miles from the area of origin and last for hours after leaving a storm.

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