Squall Line Severe Thunderstorms

When thunderstorms grow into a linear convective mode, they’re often referred to as a “squall line” of strong thunderstorms. This type of thunderstorm is most likely to bring very heavy rainfall and strong winds, and is not as likely to bring tornadoes or large hail. Squall lines are one long line of strong thunderstorms as opposed to the many discrete thunderstorms, which result in supercells. A very strong squall line can sometimes result in a storm known as a derecho having even stronger winds over a larger area, which tend to be very damaging. The tweet below is an example of radar imagery of a squall line in northeast Florida.

Squall Line Dynamics

Squall lines are lines of convective thunderstorms which feed off of warm, moist inflow. Squall lines need to continuously progress into that warm, moist area in order to have more energy to use to create convection. This is why they tend to create a damaging wind threat, because they are moving quickly in the same direction in which they are producing a gust front of wind. For example, the gust front may only be 30 miles per hour, but when you consider the entire system is also moving at 30 miles per hour, that leads to a relative wind of 60 miles per hour. Typically, winds 60 miles per hour and greater are enough to cause property damage and will be warned by the National Weather Service with a Severe Thunderstorm Warning.

Large hailstones are not very common with squall lines because the lapse rates and updrafts are not strong enough to support the formation of large pieces of hail. Squall lines tend to be very moist, which typically has a smaller lapse rate, which does not create the very strong updrafts which are needed to create large hailstones. Some squall lines can still produce tornadoes, especially along and near binds or kinks in the squall line, which causes air to circle in on itself. Squall line (or QLCS) tornadoes often happen very quickly and are difficult to predict and warn on.

Derechos

Derechos are a special kind of squall line which produce damaging winds over a large area. The main component of a derecho is the rear inflow jet, which adds wind speed to the overall line of thunderstorms. In the example image above, the rear inflow jet, which is caused by the upshear tilt of the thunderstorm, adds to the overall speed of the progressive gust front and can result in much more damaging winds than a regular squall line.

The above tweet is an example of a powerful derecho, which affected Iowa and Illinois in August of 2020. The bowing segment where some radar reflectivities move ahead and faster than others represents a relatively strong portion of the line in terms of wind speed. Measured wind speeds from this particularly strong bowing segment were over 100 miles per hour.