Gustnado

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A gustnado in southeastern Wisconsin on 4 October 2002.

A gustnado is a colloquial expression for a type of short-lived, shallow, cyclonic circulation that can form in a severe thunderstorm. While it derives its name from the tornado, it has little in common with tornadoes structurally in terms of vertical development, or in regard to intensity, longevity, and formative process (classic tornadoes are associated with mesocyclones).

The average gustnado lasts a few seconds to a few minutes, although there can be several generations and simultaneous swarms. Most have the winds of an F0 or F1 tornado, and are commonly mistaken for tornadoes. However, unlike tornadoes, the rotating column of air in a gustnado usually does not extend all the way to the base of the thundercloud. Gustnadoes actually have more in common with whirlwinds (which include dust devils, whirlwinds that form due to superheated surface layers and stretched vorticity, most commonly on sunny, warm days with light winds). They are not considered true tornadoes (unless they connect the surface to the ambient cloud base) by most meteorologists and are not included in tornado statistics. Sometimes referred to as spin-up tornadoes, that term more correctly describes the rare tornadic gustnado that connects the surface to the ambient clouded base, or to relatively brief tornadoes associated with a mesovortex.

The most common setting for a gustnado is on the outflow from a severe thunderstorm (58+ mph winds). They are triggered by gust fronts (hence the name) in thunderstorms. The cool air in the gust front acts like a mesoscale cold front, it slices under the warm air ahead of it, creating upward motions and turbulent interactions. The friction from this interaction creates a spinning column of air, or eddy, which can create a gustnado (to get the general idea of this, picture an area of leaves swirling on a windy day, just on a much larger scale).

In addition to forming on the leading edge of a thunderstorm, which in a supercell is the front-flank downdraft, gustnadoes are also not uncommon in the rear-flank downdraft of supercells. This region often contains high vorticity air, and sometimes highly buoyant air, both of which are conducive to the formation of gustnadoes as well as tornadoes. Gustnadoes near the updraft-downdraft interface may evolve into bona fide tornadoes when ingested into a mesocyclone.

Although "gustnado" has yet to enter accepted weather nomenclature, the term is popular in the Great Plains and Midwest of the United States, where the phenomenon is most common.