Outflow Boundaries

Outflow boundaries are a thing of beauty in the weather world. They occur when rain cooled air, which is more dense than the warm air around it, hits the ground and spreads out in all directions. Since the cool air cannot go into the ground, it is forced to spread out away from the ground along the surface. It is the same meteorological process that develops shelf and roll clouds which we talk about here!

Another name for an outflow boundary is a gust front which acts as a mini cold front. Once it passes, you will typically notice a wind shift and temperature change. During Saturday’s severe storms across north Iowa, a strong outflow boundary developed. They typically appear as a thin line on radar from density differences or bugs/dust getting caught up in which reflects the radar beam back to the radar.

Can you spot it? If not, here is my poor attempt at putting arrows on the gif to highlight the outflow. Notice it spans across the entire state of Iowa and into parts of Illinois and Nebraska.

Outflow boundaries can spark off new showers or storms, especially if that air is converging with another front. Sometimes these storms can be quite severe. One example is below from Alabama.

NWS Birmingham, AL – Localized Flooding of July 26, 2017

Luckily that wasn’t the case yesterday but this outflow was so strong that it prompted a severe thunderstorm warning for a few counties and caused damage to trees in Waverly even though the storm didn’t directly pass over and there was likely little to no rain.

Here were some viewer submitted photos of the damage:

Ann Seggerman
Milton Wikstrom
Ann Seggerman

To close here is an interesting feature that we noticed on radar from July 17th near Omaha. Showers and storms moved through to the north of Omaha in the morning and developed an outflow boundary. This boundary slowly crept north until the low level winds were enough to push it further north. At this point the boundary was analyzed as a stationary front.

Brandon Libby

Brandon Libby

Meteorologist
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