Squall Lines 101: How They Form and Why They’re Dangerous

When it comes to severe weather, a classic severe weather event is a squall line. A line of thunderstorms that can stretch hundreds of miles and bring fairly strong winds, lots of lightning, hail and even tornadoes as it races, typically, eastward. Let’s dissect how a squall line forms, how it maintains itself and why sometimes we see tornado warnings on the leading edge of the line of storms.

Squall Line Beginning To Form, June 13th 2017

A low pressure system is usually behind the formation of squall line. When a low pressure system move into an area, say the Sioux Empire, an unstable and moist airmass is brought overhead at first. This airmass is typically has southerly winds meaning there’s more moisture as well as more instability than the airmass before it. Marked by the passing of the warm front, the area, the Sioux Empire, then waits for the low to exit… by the passing of a cold front and the formation of a line of storms – the squall line.

Dew Points Before The Warm Front Passed

Expected Dew Points After Warm Front Passes

Marked by northwesterly winds, the cold front follows behind the main circulation of a low pressure system. As it passes over an area, drier, cooler and more stable air replaces the unstable air ahead of it. Because cooler and drier air is more dense, as the cold front makes its way through an unstable airmass, it forces that moist air upwards. Think of it like a putty knife scraping off paint where the cold front is the putty knife and the paint is the moist air; as you slide the knife through the paint, it’s forced upwards. That’s exactly what happens when a cold front, especially a strong one, moves through a moist airmass.

Cold Front Moving Through The Sioux Empire

As the cold front begins to cut through a moist airmass, a line of storms typically begins to form just ahead and  parallel to the actual cold front. Usually it’ll start off with a couple discrete cells; sometimes these cells become tornadic as winds ahead of the actual cold front are still out of the south however; there’s turning in the atmosphere the higher you go as winds turn back out of the west/northwest. Outflow from these discrete cells, in addition to the cold front moving eastward, help further create the squall line. Watch carefully the next time either myself, Brandon or Kole are covering a squall line on air, you’ll see that the line will not only maintains itself as it moves east (since there’s untouched, unstable air east of the line) but that the line generally grows at the southern point.

Squall Line Beginning to Form, August 2016


Southern End of Expected Squall Line Stronger Than Northern Cells, July 2016

As a squall line moves east, those in the path will most likely get hit by strong winds, hail as well as experience frequent lightning and heavy rains. Sometimes during a squall line event, portions of the line will bow out called a bow echo; named because it takes the similar shape of an archers bow as rain cooled air behind it forces the leading edge to bow out. Bow echoes can sometimes cause straight line winds and even tornadoes.

Tornadoes typically form on the edges of these bow echoes in a place that we call bookend vortices. These spin-up tornadoes are not as strong as tornadoes that we find in discrete cells for one very important reason – they do not have a mesocyclone. Unlike in discrete supercells, the rotation associated with tornadoes on a squall line do not extend through the column of air. In a line of storms, the farther up in the atmosphere you travel, the winds are out of the northwest and more linear (meaning there’s less turning).
Even though the spin-up tornadoes are not as strong and typically are gone before the tornado warning is allowed to expire, that doesn’t mean they aren’t any less dangerous. Please take these tornado warnings as seriously as you would if they weren’t associated with a line of storms. We want to make sure that you are safe during every severe weather event.

Tornado Warned Cells on Leading Edge of Squall Line, June 2107

The Same Area As Photoed Above, Now Using Velocity Scan To Show Weak Rotation

In extreme cases, the whole squall line can take a bow like shape and become a derecho. Occurring typically during the summer months of June, July and August, these events can travel hundreds of miles as well as extend hundreds of miles, be long lived and bring, in some cases, hurricane strength wind due to high instability as well as strong, vertical wind shear. Typically the KDLT forecast area sees about one derecho every two to four years while parts of the lower Midwest/Missouri area see one derecho every year.

Derecho Climatology, By Dennis Cain

Hopefully we skip out on the derecho threat this year but if one were to occur or a squall line develops and races across the forecast area, after reading this, you’ll know a little more about what’s causing them, what’s occurring in the upper atmosphere as it passes and why sometimes we see tornado warnings issued with these lines.

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