Squall Lines Are A Serious Danger That Can Pack Damaging Winds, Tornadoes

Squall line is a common term you might hear meteorologists say when there is a threat of severe thunderstorms in your area.

Here's what you should know about squall lines and why you should take them seriously.

W​hat 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 about 10 to 20 miles wide.

Here's a forecast model depiction showing what a squall line typically looks like on radar. Keep in mind, the squall line can be broken into two or more parts, and sometimes severe thunderstorms called supercells can form ahead of it.

3 Reasons W​hy Squall Lines Are So Dangerous

1. Nearly all squall lines have destructive straight-line winds in them, but some are more intense than others. The strength of the winds depends on variables like how unstable the atmosphere is, and the strength of winds above ground level.

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. The wind can worsen the damage from large hail since it accelerates the hailstones into vehicles and the siding of homes.

A derecho is a type of squall line that meets certain criteria. These long-lasting squall lines can spawn gusts over 100 mph, the equivalent of an EF1 tornado, in extreme cases.

2. Tornadoes are also a common threat found in squall lines. Sometimes tornadoes develop quickly along the leading edge of a squall line of severe thunderstorms with otherwise damaging straight-line winds.

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

Below 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.

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

3. Blinding Rain and dangerous lightning are concerns. Extreme rainfall rates of an inch or more per hour can occur in a 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 avoided when a squall line hits, but never use a highway underpass as shelter.

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.

Chris Dolce has been a senior meteorologist with weather.com for over 10 years after beginning his career with The Weather Channel in the early 2000s.