05/16/2025
~Weather Classroom: Squall Lines and Derechos~
Given the crazy winds from the squall line last night, I though I'd harken back to the August 10th, 2020 derecho (which is what both images in this post are from) that left many of us without power for several days and do crash course on squall lines and derechos!
Anyone who's been following me for a while has heard me use the term 'squall line' as well as the acronym 'QLCS' with regard to 'QLCS tornadoes.' But what is a squall line?
A squall line—properly named a quasi-linear convective system—is a long line of organized storms that can span multiple hundreds of miles. They typically form ahead of cold fronts (to the point that they were often referred to simply as "cold fronts" for a very long time) and bring with them damaging-to-severely-damaging winds, medium to very large hail, tons of lightning, heavy rainfall (not always), and sometimes QLCS/squall line tornadoes.
Squall lines will also often outrun the cold fronts that create them as the strong winds caused by a line of cold air smacking into much warmer air acts similarly to putting a car's gas pedal to the floor.
You'll often hear TV meteorologists use the term 'bow echo' when discussing weather radars; a 'bow echo' is another term for a squall line as it refers to the bow shape of the storm system, with 'echo' referring to the radar itself, as radar is an 'echo' of the radio waves that get sent into the atmosphere to give us the weather radar images. When you hear a forecaster use the terms 'bow echo' or 'squall line,' be prepared for strong winds!
Derechos are one of the strongest, longest-lived manifestations of squall lines and can be some of the fastest-moving non-tropical storms in the world with travel speeds capable of reaching 70mph, and can cause wind gusts over 100mph!
In fact, derechos have been known to cause *more* damage than hurricanes and tornadoes as sudden, powerful, straight-line winds can easily rip off roofs, blow down weaker structures, blow over trees, and blow out windows in buildings with breathtaking ease, to say nothing of the damage done to homes and vehicles by trees and large limbs landing on them. The damage can also be significantly more widespread as derechos are so big and travel so far. Derechos on average tend to travel for about 250 miles, but the 8/10/2020 derecho traveled an astonishing 770 miles in just 14 hours!
I mentioned 'QLCS tornadoes' up there, too, didn't I?
QLCS tornadoes, aka squall line tornadoes, are tornadoes that are typically short-lived and relatively weak that are created by squall lines; they also have a tendency to hop-scotch—they'll lift, then drop, then lift, then drop. While most QLCS tornadoes are, as I said, fairly weak, there have been some notable instances of EF3 and EF4 QLCS tornadoes hitting—but it's worth noting that the EF rating is less about wind speed and more about damage. A tornado with very high wind speeds that drops in the middle of nowhere can still be rated EF0, whereas a weaker tornado that drops in the middle of a city can wind up with an EF3–EF4 rating due to property damage.
A great example of QLCS tornadoes are waterspouts; there have been a lot of great videos of them over Lake Michigan over the years as squall lines blow through!
Derechos are well known for spawning multiple QLCS tornadoes, however the biggest threat with them will always be the powerful, straight-line winds.
This concludes our Weather Classroom squall line crash course! If you have any questions, do feel free to ask!
