What are QLCS tornadoes, and why are they so dangerous?
Tornadoes typically form from large thunderstorms, but recent research indicates that they can happen during lines of storms as well.
The tornado that hit Chicago was stronger than most tornadoes and much harder to warn people about than a typical supercell tornado.
Tornadoes come in all shapes and sizes; however, a lesser-known but equally menacing phenomenon is a type of fast-developing twister that can be difficult to see, which makes it even more dangerous.
Tornadoes are vortexes of wind that are connected to the base of a thunderstorm, most often an individual supercell thunderstorm. Recent studies have shown that a different kind of tornado can also occur during a line of storms that contains heavy rain and high winds.
A Quasi-Linear Convective System (QLCS), is a line of storms, as opposed to an individual thunderstorm. QLCS can spawn tornadoes although they are often weaker than those formed from a discrete cell storm.
A radar image showing a line of storms that developed a QLCS (blue circle) on the north side of one of the bow echoes (white lines) in the line of storms approaching Chicago on June 20, 2021.
According to a study by Iowa State University, QLCS tornadoes typically form on the northern side of a bow echo, a portion of the line of storms that moves faster than the other parts of the line, bulging out in front. This can create a circulation on the northern edge that can turn into a tornado.
QLCS tornadoes represent 20 percent of all tornadoes and are more likely to occur overnight. They are also harder to detect on radar and harder to observe and document, as they are often near heavy rain. These tornadoes can be fast to spin up, minimizing warning lead time and making them one of the more dangerous types of tornadoes.
On June 20, 2021, an unusually strong QLCS tornado, ranked EF3 on the Fujita Scale, tracked through the suburbs of Chicago for 17 miles, causing massive destruction along the way.
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