You've probably heard of a squall line and you know it's bad. But what is it and why does it form?
A squall line is line of broken to continuous thunderstorms - which means you can't fly through the line. They form ahead of cold fronts in warm air, and develop into a multi-cell thunderstorm line.
So why does a squall line become so strong - and why can it last far longer than a typical thunderstorm? The key is in how a typical storm operates in it's mature stage - and why it dies out.
The Three Stages Of A Thunderstorm
Cumulous Stage
During the cumulous phase, the cumulous cloud sucks air in from its base, which rises up forming a towering cumulous cloud (TCU).
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Mature Stage
During the mature stage, rain forms in the cloud, cooling the air and causing a downdraft. You'll also see lightning and hear thunder in this stage. The updraft feeding the thunderstorm exists side-by-side with the downdraft, meaning they don't mix. The thunderstorm can continue to grow and strengthen because air continues to flow in.
Dissipating Stage
During the dissipating stage, the rain and downdrafts mix into the path of the updrafts - cutting off the thunderstorm's supply of energy. Once this happens, the thunderstorm stops growing and begins to fall apart.
Multicell Storms Keep Growing
Squall lines develop as multi-cell storms. Their structure allows them to keep feeding updrafts into the storm as the downdrafts pour out.
The Gust Front
The gust front is one of the secrets to a squall line's longevity. As a squall line develops into the mature stage, the downdrafts form a gust front, pushing cold, fast moving air out ahead of the storms. That lifts up the warmer air in front of the gust front, which the thunderstorms suck in through updrafts. This keeps the updrafts and downdrafts separate - meaning the storm keeps growing.
The Outflow Boundary
Gust fronts from squall lines can push massive amounts of cold air ahead of the storms. Eventually, this air can form an "outflow boundary," essentially a wall of cold air moving ahead of the storms. This wall is so powerful that it can travel for hundreds of miles and last as much as a day after the storm front dissipates.
An Example - The Colorado Squall Line On June 8th
A great example of a squall line formed on the afternoon of June 8th, ahead of a cold front.
At 1525Z, you can see that the radar over Colorado is mostly clear. There's a low pressure system sitting over the north central part of the state.
At 1725Z, you can see a line of storms begin to form. Looking at the surface analysis for the same time, you can see the low has connected to another low in the southern part of the state via a low-pressure trough.
By 1925Z, a strong line of thunderstorms forms. By 2125Z, the line strengthens and a cold front forms between the two lows.
At 2325Z, the thunderstorms form a continuous line, moving southeast across Colorado.
By 0000Z on the 9th, the surface analysis clearly shows a cold front moving across southeastern Colorado. Radar shows the line of thunderstorms moving ahead of it into northwestern Texas.
At 0300Z, the surface analysis shows a squall line rolling across north Texas. Radar confirms that the squall line extends from the low in southwestern Nebraska all the way through north Texas.
These storms continue to strengthen and move ahead of the cold front.
At 0900Z, the surface analysis shows that an outflow boundary has formed and is moving through central Texas. Radar shows that the squall line has broken up, and is now a cluster of thunderstorms in central Texas.
As the morning progresses, this outflow boundary moves southeast towards the Gulf of Mexico, pushing the cluster of storms with it.
It's Nearly Summer - Find Your Own Example (On The Ground, Please!)
With summer almost here, you'll have countless opportunities to find and track squall lines - just do it from the ground! Keep an eye on the prognostic charts and surface analysis and see if you can recognize conditions where they can form.
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