Outflow Boundaries and Gust Fronts
by Daphne Thompson, on Jun 2, 2017 1:59:49 PM
We have all experienced a gust front, or outflow boundary, before. It is a process associated with thunderstorms and can cause winds strong enough to cause damage. While tied to thunderstorms, the gust front gets to you before the storm does.
Gust fronts and outflow boundaries are the same thing. Similar to a cold front, they separate the thunderstorm cooled air from the rest of the environment. If you are outside when a gust front passes, you will immediately note a change in the direction of the wind or possibly even a strong wind when it was calm prior. As it passes, the temperature will also drop.
As a thunderstorm forms, there is both an updraft of warm air and a downdraft of cold air. Once established, the rain falling from the storm will push out cold air ahead of it. This can cause major issues for aircraft taking off or landing due to the wind shear. The process is better understood now, and pilots are better trained on how to handle the condition.
Visually, a gust front may appear as just a line of clouds quickly approaching. They are associated with shelf and roll clouds, which are a type of arcus formation. While very dramatic looking, these clouds do not signal any type of tornadic activity. They do, however, signal the approach of cold wind. Many times, rain will also follow.
On radar, a gust front can usually be seen as a thin line of reflectivity moving away from the storm. This reflectivity signature may be caused by convection, but can also be the result of bugs getting caught in the front or dirt being kicked up with the winds. If two outflow boundaries collide, you may see additional storms form at the intersection. In fairly calm conditions, a storm may create a circular outflow. You can see that occurring in the RadarScope image below.
It is similar to how a downburst, or microburst, may look. In a downburst, the outflow is caused by the storm actually collapsing. Imagine dropping a water balloon on a hard surface. As it hits, the water will go out in all directions from the center. A downburst was responsible for the crash of the 1985 Delta Air Lines Flight 191 in Dallas and may also have played a part in the collapse of the Dallas Cowboy's practice facility in 2009.
Most gust fronts will not contain very strong winds, but many times the larger ones will have severe, damaging winds. Some have even been the cause of fatalities, as what happened when the Indiana State Fair stage collapsed prior to a concert by Sugarland in 2011. Gustnadoes can form when a gust front creates a small eddy. While not tornadic in nature, a gustnado can create EF-0 to EF-1 damage.
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