One of the nicest things about summer is a nearby afternoon thunderstorm. The storm bubbles up on the horizon and a few minutes later you get a nice gust of cool, refreshing wind. This gust of wind is called an "outflow boundary," and it's one of the most interesting weather phenomena there is.
Most summertime thunderstorms are known as "single cell," or individual storms that bubble up, dump heavy rain and lightning for half an hour, then collapse. During the early stages of a thunderstorm's life cycle, its updraft (the warm, moist air flowing into the storm) feeds the structure the energy it needs to allow it to grow in size and strength.
Eventually, the weight of the water in the thunderstorm will become too great for the updraft to sustain, causing precipitation to fall and a downdraft to develop. The downdraft, or the cooler air that sinks out of the thunderstorm, will pool up on and above the ground beneath the storm in what's known as a "cold pool." This cold pool will begin spreading out away from the storm like a ripple on a pond, cutting off the updraft and starving the storm of its fuel, causing it to dissipate.
When the cold pool starts radiating out away from the parent thunderstorm, it's often called an "outflow boundary." The outflow boundary acts like a mini cold front, bringing with it cooler temperatures and gusty winds. Oftentimes, these outflow boundaries are able to force the warm, moist air ahead of them to rise, triggering more thunderstorm activity. This often happens on the Gulf Coast; a couple of storms will form and dissipate, sending out outflow boundaries that trigger a domino effect of thunderstorms that continues until the sun sets and the instability subsides.
While these pop-up storms are the most familiar, all types of thunderstorms — single cell, multicell (clusters or lines), and supercells — all produce outflow boundaries.
We can often see these outflow boundaries on Doppler weather radar thanks to two major factors: density differences and bugs. Warm air is less dense than cold air, so there's a change in density along the outflow boundary. This density difference can reflect the beam back to the site, appearing as a fine line on radar imagery. Clouds of bugs and dust can also get caught along the leading edge of an outflow boundary, which also reflects back to the radar site as a return.
The above radar image is from Chicago this past Monday. Thunderstorm activity in the area produced some very visible outflow boundaries on Doppler radar. If you can't spot them all, here they are highlighted in white:
You'll notice that two outflow boundaries collided with each other between Kankakee and Pontiac, creating enhanced lift and triggering a small but intense thunderstorm just west of Kankakee.
The best spot in the country to see outflow boundaries on weather radar is in Mobile, Alabama, which sits smack in the middle of the northern Gulf Coast. Mobile is the wettest city in the country thanks to its almost-daily thunderstorm activity during the summer, and the radar is almost as fun to watch as it is to experience the storms themselves.
Here's a snapshot of some storm activity in Mobile from September 16, 2013, showing nearly a dozen outflow boundaries:
There are more boundaries than what we can see on the radar — the radar eventually stops detecting these features because the beam gets higher off the ground the farther it gets from the radar site.
It's not just individual storms that produce outflows. Lines of thunderstorms develop along the leading edge of a well-developed cold pool. The most organized lines of storms are called mesoscale convective systems (MCS), and an MCS basically creates its own outflow boundary to sustain itself. You can see some very well-defined outflow boundaries on radar during these situations, especially after a particularly intense derecho starts to weaken.
A great example of an outflow boundary ahead of a line of storms is the Ohio-D.C. Derecho of 2012. When the derecho crossed the Appalachian Mountains, the northern half of the derecho kept up with the cold pool, allowing the storms to remain severe as it walloped the D.C. and Baltimore areas. The southern half of the derecho rapidly weakened after the cold pool started moving faster than the storms, cutting off the updrafts and forcing the storms to dissipate.
Even though the storms began to dissipate, the cold pool kept on racing to the southeast, appearing on radar as a well-defined outflow boundary.
The next time there are storms in the area, check out the radar and look for these awesome features, and be sure to look up towards the sky. Not only are outflow boundaries pretty cool (figuratively and literally), but the clouds they produce are often spectacular as well.
[Top image by the author, radar images via Gibson Ridge]