What is a Squall Line?

These lines of severe thunderstorms are a common occurrence during severe weather season
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By TWSE Explains


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A squall line is a line of strong and severe thunderstorms common in the spring and summer and often produce strong winds, lightning, heavy rain, and hail. Squall lines often form ahead of cold fronts but can develop independently if several strong thunderstorms merge.

Squall lines often damage property since they cover such a large area. This blog explains what they are and what you should do.

What is a squall line?

A squall line is a band of thunderstorms, which can be hundreds of miles long. These lines typically form along the leading edge of a strong cold front, where there is a sharp temperature difference ahead and behind the front.

As the two air masses collide, they create an area of instability as the cold air scours out the warm air, creating lift. As the air rises, it cools and condenses, leading to the formation of clouds and later thunderstorms.

A squall line is easily identifiable on radar by a long stretch of stronger returns, typically colored orange and red on most radar images. On another doppler radar image called base velocity, the squall line is often clearly defined by a shift in wind direction.

Squall lines may also ‘bow out,’ otherwise known as a bow echo. A bow echo is a radar echo that often appears as an arc or bow-shaped feature in the reflectivity image. It is caused by a combination of strong updrafts and wind shear and can result in very severe weather, often in the form of strong winds.

The series of images below show this process.

Bow Echo 1Image Credit: NOAA/NWS JetStream
Bow echo 2Image Credit: NOAA/NWS JetStream
bow echo 3Image Credit: NOAA/NWS JetStream

The dangers associated with squall lines

As opposed to discrete storms, squall lines most commonly produce heavy rain, hail, and strong winds. Tornadoes are less frequent with a squall line, and if they do happen, they’re often weak and short-lived.

The most dangerous aspect of squall lines is their potential to create severe straight-line winds. In the most severe squall lines, gusts can approach 100 mph. These winds can easily damage homes and down trees, and power lines can block roads and disrupt power supplies. In addition, heavy rains can lead to flash flooding, which can quickly turn deadly.

One positive to squall lines is that they’re often short-lived thanks to their quick movement. Often the worst weather is over in minutes, although the cleanup afterward may take days or weeks.

Derechos: Squall lines on steroids

A derecho is a widespread, long-lived windstorm associated with a band of rapidly moving severe thunderstorms identified on radar by the presence of a well-defined bow echo. Although they are most common in the Midwest and Great Plains, derechos can occur anywhere in the United States.

Derechos are most common during the summer months when warm air from the Gulf of Mexico collides with cooler air from the Rockies. However, they can also occur in the spring and fall.

Derechos are unusual because they can produce hurricane-force winds over a large area, often exceeding 250 miles. As a result, they can cause widespread damage, particularly to trees and power lines. In addition, derechos may come as part of multiple waves of severe weather.

derechoImage Credit: JSirlin - stock.adobe.com

What to do if you are caught in a squall line

If you find yourself caught in a squall line, the most important thing to remember is to stay safe. Here are some tips on what to do if you find yourself caught in a squall line:

  • If possible, seek shelter inside a sturdy building. Shelter inside a brick building if possible, and in interior rooms. Stay away from pole buildings and pavilions.
  • Avoid being outside in open areas. If you are caught outside, try to find a low spot away from trees and power lines. Be alert for rapidly rising water, and protect your head.
  • Stay away from windows and doors. Doors may break, and windows can shatter due to strong winds and flying debris.
  • If you are driving, stay in your car. Pull off the road and into a parking lot or other open area. Do not attempt to exit the vehicle, and do not park under bridges or overpasses.
  • Be alert for flash flooding. If you see water beginning to rise rapidly, seek higher ground.
  • Have a way out if need be. Be aware of your surroundings and watch for falling debris. You may need to move to protect yourself. Protect your head at all times, and use a mattress or pillow to protect the rest of your body.

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TWSE Explains

Articles written by The Weather Station Experts staff to help break down even the most complex weather topics.