Tornadoes vary in size and shape, but there is a less well-known yet equally dangerous type that develops rapidly and can be hard to spot, increasing its risk. These twisters are vortexes of wind that form at the base of thunderstorms, typically supercells. Recent research has revealed a different kind of tornado that can occur within a line of storms characterized by heavy rain and strong winds.
Known as a Quasi-Linear Convective System (QLCS), this type of storm consists of a line of storms rather than a single thunderstorm. While tornadoes can still form within a QLCS, they tend to be weaker compared to those spawned by individual supercells. A radar image from June 20, 2021, captured a QLCS developing on the northern side of a bow echo within a line of storms approaching Chicago.
Research from Iowa State University indicates that QLCS tornadoes typically form on the northern side of a bow echo, which is a part of the storm line that moves faster, creating a bulge in the front. This bulge can generate a circulation on the northern edge that leads to tornado formation. QLCS tornadoes make up about 20% of all tornadoes and are more likely to occur during nighttime. They are challenging to detect on radar and observe due to their proximity to heavy rain, making them particularly dangerous.
On June 20, 2021, an especially powerful QLCS tornado, rated EF3 on the Fujita Scale, tore through the suburbs of Chicago, leaving a trail of destruction spanning 17 miles.
