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https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/04/07/how-thursdays-tornadoes-in-the-washington-area-formed/?utm_term=.a0441ac2afc5When people think tornado, the first thing that comes to mind is the strong or violent tornado of the Great Plains, generated by supercell thunderstorms. But nature manufactures tornadoes in many ways, and another relatively ubiquitous type are “squall line tornadoes,” generated along the front edge of a bowing or arc-shaped complex of thunderstorms.

According to one research study, these types of squall lines, called quasi-linear convective systems (QLCS), generate 20 percent of all tornadoes in the U.S. They are typically weaker, smaller and shorter-lived than the classic supercell-type tornado, which is manufactured by a spiral or “helical” updraft called a mesocyclone.

This was a very curious line of storms, for several reasons. For one, it lacked lightning. Two, it had a very wavy structure, with multiple arc-shaped segments separated by sharp indentations or notches. Three, it formed in an environment that was only marginally unstable, but that lack of instability was compensated by extremely vigorous ascent of air beneath the jet stream.

A packet of upper-atmosphere spin energy, called a vorticity maximum, was surging into the Washington region, and the squall line was the direct reflection of violently rising air along the leading edge of this energy packet. See more…