Tornado Central

What A Squall Line Is And 5 Dangers They Pose, Including Destructive Straight-Line Winds, Tornadoes

These dangerous lines of severe thunderstorms are common in spring and other times of the year and come with multiple threats.

Potential Severe Outbreak Ahead, Tornadoes Possible

When severe weather strikes, you might hear a meteorologist refer to a squall line. They are commonly seen in spring as well as other times of the year and you should take the dangers they pose seriously for multiple reasons.

Here's everything to know about them and the five threats they produce.

Big Picture: Defining A Squall Line

Squall lines are thunderstorms arranged in a line, often from north to south. They can be hundreds of miles long but are usually only about 10 to 20 miles wide.

The image below shows an example of what a squall line looks like on Doppler radar, where the narrow band of red and dark orange echoes creates the "line" of squall weather. Keep in mind, that the squall line can be broken into two or more parts, and sometimes severe thunderstorms called supercells can form ahead of it.

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Example radar image of a squall line. — Example radar depiction of a squall line.

Deeper Dive: 5 Dangers Squall Lines Pose

1. Destructive Straight-Line Winds: Stronger squall lines commonly produce wind gusts from 60 to 80 mph. Winds this strong are capable of downing tree limbs and knocking out power at the very least, but they may also be strong enough to down trees onto homes, vehicles or anything else in their way. Those winds are also strong enough to produce damage to homes and other buildings.

When the National Weather Service issues a severe thunderstorm warning for dangerous winds in a squall line, you should seek shelter as you would when a tornado warning is issued.

2. Tornadoes: Sometimes tornadoes develop quickly along the leading edge of a squall line of severe thunderstorms with otherwise damaging straight-line winds. Because this type of tornado can spin up rather abruptly, it can be a challenge to issue tornado warnings well in advance.

These tornadoes may occur overnight or be rain-wrapped and difficult to see.

Tornadoes can also sometimes form in advance of a squall line if supercell thunderstorms precede one.

This is a Doppler radar image showing an EF2 tornado that developed in a squall line and struck Mount Olive, Mississippi, on Jan. 2, 2017. The small notch just below the Mount Olive label depicts the rotation in the squall line indicating where the tornado was on the ground for just over 4 miles.
(National Weather Service Jackson, Mississippi)

3. Wind-Driven Hail: Squall lines are often capable of producing severe hail, defined as 1 inch in diameter – roughly quarter-size – or larger. High winds in squall lines can worsen the damage from the hail since it accelerates the hailstones into vehicles and homes.

If an incoming squall line has a history of producing severe hail, move your vehicles into a garage, parking deck or some covered area, if possible. However, if you are driving, do not park under a highway overpass. This could cause a backup on the highway if many people attempt to do the same thing, placing many more people at risk of an incoming severe thunderstorm.

4. Blinding Rain: Extreme rainfall rates of an inch or more per hour can occur in squall lines.

If you are driving when the heavy rain hits, visibility can drop to just a few feet in front of your car in a matter of seconds. Travel should be paused when a squall line hits.

5. Dangerous Lightning: Thousands of cloud-to-ground lightning strikes per hour can also occur in a squall line. Occasional lightning strikes can continue for a half-hour or longer after the squall line and the highest winds have already passed.

With lightning nearby, avoid contact with electrical devices, corded phones and metal pipes while indoors. It is safe to go outdoors 30 minutes after hearing the last clap of thunder.

Lightning over the Great Plains near Childress, Texas, in May 2009.
(NOAA Photo Library)

Geek Out More: The Most Extreme Type Of Squall Line

Another term you might have heard is a derecho, which is a type of squall line that meets certain criteria.

From the Spanish word for "straight," these windstorms leave wide, long areas of straight-line wind damage. The winds can be as strong as 60 to 100 mph or higher in extreme cases.

To be called a derecho, the thunderstorm-generated high winds and wind damage should cover a distance of at least 400 miles long and be at least 60 miles wide, according to a 2016 proposal led by Stephen Corfidi.

(MORE: Everything To Know About Derechos)

The most extreme derechos can turn into disasters that cause billions of dollars in damage. A Midwest derecho in August 2020 caused $13.5 billion in damage when adjusted for inflation.

Above: An extreme example of a derecho producing destructive straight-line winds in a squall line happened in the Midwest on Aug. 10, 2020. In 14 hours, the derecho caused an estimated $13.5 billion in damage across the 770 miles it traveled. Four deaths were blamed on the storm complex.

Chris Dolce has been a senior meteorologist with weather.com for nearly 15 years after beginning his career with The Weather Channel in the early 2000s.