Outflow Boundaries and Thunderstorm Development

Ben Baranowski
Jul 27, 2017 · 2 min read

If you look at enough weather radar data in the summer, you’ll see storms surrounded by circular lines. Below is a Radarscope screenshot using the annotation tool to show the circular lines.

A thunderstorm generated outflow boundary. The boundary is identified by the blue arrows.

These lines are thunderstorm outflow — rain cooled air from the core of the storm that’s transported to the surface by the precipitation. The cooler air hits the ground and spreads out. In many cases, there are gusty winds within the outflow that loft bugs, dirt, grass, dust etc. that the radar can see and shows up as fine-lines.

Outflow boundaries can also act as local foci for new thunderstorms. In the screenshot above, this is indeed the case. However, you’ll notice that not all areas of the outflow boundary are creating new thunderstorms. Why is this? The answer lies in wind shear — the change in wind speed, direction or both.

The interaction of wind shear with the outflow boundary can enhance or hinder areas for thunderstorm development. Below is a screen shot of low-level wind shear over the Gulf states. Notice that the shear barbs in Central Alabama are generally pointing south. Therefore, the down shear side of the outflow boundary would be the southern side.

Low level wind shear barbs

So if new thunderstorms develop on the outflow boundary, we would expect to see them on the southern side. Below is an animation of the outflow boundary and the new thunderstorms that form.

The initial storms near Hoover create the outflow that spread out and shortly after, storms form on the west and southern sides of the boundary.

About 40 minutes later, most of the western storms have died while more storms continue to form on the southern end of the outflow boundary, just as we would expect!

Expanding Outflow Boundary

The next time you see an outflow boundary on radar, turn on the animation and see if there is a preferred location for new storm development.

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