Here’s why storm lines pose a serious danger

A squall line is a common term you may hear from meteorologists when there is a threat of severe thunderstorms in your area.

Severe weather from Thursday night into Friday in the South could be another example of this dangerous phenomenon. Here’s what you should know about squall lines and why you should take them seriously.

What is 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 10 to 20 miles wide.

Below is a forecast model depiction of what a squall line typically looks like on radar. Keep in mind that a squall line can split into two or more parts, and sometimes severe thunderstorms called supercells can form in front of it.

Why are they dangerous?

1. Destructive straight line winds

Almost all squall lines have strong winds, but some are more intense than others. The strength of the wind depends on variables such as the extent of the instability of the atmosphere and the strength of the wind above ground level.

The strongest squall lines typically produce wind gusts of 60 to 80 mph.

Winds this strong are capable of knocking down tree branches and cutting power at the very least, but they can also be strong enough to topple trees onto homes, vehicles, or anything else in their path. These winds are also strong enough to cause damage to homes and other buildings. Winds can exacerbate damage from large hail because they accelerate hailstones onto vehicles and the sides of homes.

A derecho is a type of squall line that meets certain criteria and can generate wind gusts in excess of 100 mph, the equivalent of an EF1 tornado, in extreme cases.

An extreme example of producing destructive straight-line winds occurred in the Midwest on August 10, 2020. In 14 hours, the hurricane caused an estimated $11.5 billion in damage across its 770 miles. Four deaths were blamed on the storm complex.

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

Tornadoes are also a common threat found in squall lines.

Sometimes short tornadoes develop quickly along the leading edge of a squall line of severe thunderstorms with damaging straight-line winds.

These tornadoes may occur overnight or be enveloped in rain and difficult to see.

Many tornadoes in the squall line are rated as EF0 or EF1. But sometimes they can be powerful and produce EF2 damage or worse.

Below is a Doppler radar image showing an EF2 tornado that developed in a squall line and struck Mount Olive, Mississippi, on January 2, 2017. The small notch at the bottom of the Mount Olive label depicts the rotation in the squall line indicating where the tornado was on the ground a little over 4 miles away. .

3. Heavy rain and dangerous lightning

Heavy rainfall rates of an inch or more per hour can occur in storm lines.

If you’re driving during heavy rain, visibility may drop to just a few feet in front of your car within seconds. You should avoid traveling during a storm line, but never use a highway underpass for shelter.

Thousands of cloud-to-ground lightning strikes per hour can also occur in a storm line. Occasional lightning strikes can last for a half hour or more after the squall line and high winds have already passed.

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

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment, and the importance of science in our lives. This story does not necessarily represent the position of our parent company, IBM.