DISCUSSION: Outflow Boundaries, they play an important role in the formation and sustainability of a thunderstorm. But what exactly is this weather phenomenon and how does it form? It all starts with the energy supplier of a thunderstorm known as the updraft. The updraft feeds the storm warm, moist air from the surface to allow it to grow in size and strength. Overtime, precipitation will begin to form inside the thunderstorm as it continues to grow. However, there will be a point at which the weight of the precipitation will become too heavy for the updraft to withstand. This will cause the precipitation and the cold air from higher up in the thunderstorm to fall towards the surface.
This is what is known as the downdraft of the thunderstorm. The cold air associated with the downdraft will hit the surface, spread out, and push the warm, moist air away from the storm. This is called the outflow or also known as the “gust front” and it essentially acts just like a cold front. It brings in cooler air to the surrounding environment as it displaces the warm moist air that was there before.
In the radar image below, the light bluish-green line surrounding the thunderstorms is the outflow boundary. There are a few ways that a radar can scan an outflow boundary. The first is due to the change in the density or weight of the air. Scientifically, we know that warm air is less dense than cold air, so the density difference will appear ahead of the storm on radar. The second way is by the objects that an outflow boundary picks up as it moves away from the “parent” storm. An outflow boundary can pick up dust particles and insects which will give a similar image to the radar scan below.
Whether the storm continues to thrive or dissipates is all dependent on the speed of the outflow boundary. If the outflow is moving faster than the storm, it will deplete the storm of its energy source and the storm will slowly start to dissipate. However, if the storm can stay near the outflow, the storm will still be able to continue to live until it doesn’t have any more energy supply left.
What’s interesting about outflow boundaries is that they can also trigger new thunderstorms around the “parent” thunderstorm. In this case, the parent thunderstorm is the storm that produced the outflow boundary. When an outflow boundary interacts with another boundary, it creates a lifting mechanism and new “baby” thunderstorms can form around the parent. A second way that the outflow can generate new storms is with its ability to act like a cold front. The cold air pushes the warm air up into the atmosphere. The newly displaced warm air acts as a lifting mechanism and prepares the environment for thunderstorm development.
Now, that’s just for just a general thunderstorm, but they aren’t the only type of thunderstorm that can produce an outflow. An organized line of thunderstorms or what is known as a Mesoscale Convective System (MCS) also uses the outflow boundary to sustain itself.
Using the radar image above, the storms on the left side will continue to strengthen and be severe warned. These storms will continue to have the capability to produce severe weather such as damaging wind gusts and small sized hail. All because the outflow boundary is not that far away from the line of storms as it will continue to supply them with the warm, moist air. While the storms on the right side of the system will eventually become too weak as the updrafts get cut off from its energy supply. These storms will eventually begin to weaken and slowly dissipate as the distance between the line of storms and the outflow boundary is quite large.
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© 2018 Meteorologist Joey Marino