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Why wind shear matters for supercell thunderstorms

Ben Domensino, Tuesday November 20, 2018 - 11:27 AEDT

Severe thunderstorms are likely to affect a number of states and territories today and supercell thunderstorms are possible in parts of western NSW.

So, what are supercells and what makes them so dangerous?

While all thunderstorms cause lightning, only a small proportion are considered severe. The Bureau of Meteorology issues severe thunderstorm warnings when there is a risk of damaging winds, heavy rain, large hail. However, even in this select group of dangerous thunderstorms, some are more intense than others.

The most destructive and dangerous type of thunderstorms are called supercells. These storms can produce destructive winds, very large hail, intense flash flooding and sometimes tornadoes.

All thunderstorms require three key ingredients to form:

An unstable atmosphere

Moisture in the atmosphere

A trigger mechanism that causes air to rise from the surface

When any of these three ingredients are available in greater quantities, thunderstorms have a better chance of becoming severe.

Supercell thunderstorms, however, require an extra ingredient to get going: wind shear.

Wind shear refers to a change in wind speed and direction with height as you move upwards through the atmosphere. Of particular interest for supercell development is the rate at which wind speeds change in the lowest six kilometres of our atmosphere.

When wind shear is high enough, meaning there's a great enough change in speed and direction with height, thunderstorms start to rotate. This rotation is an important feature of supercell thunderstorms because it allows them to continually suck in energy from the surrounding environment without collapsing in on themselves. Regular thunderstorms often die when the rising air within them gets too heavy from rain and hail, causing them to collapse. Rotating supercells displace the rising air (updraft) from the falling rain/hail (downdraft), which allows them to grow more intense and last longer.

Regular thunderstorms usually last for tens of minutes, while supercells can go on for hours. Their rotation also allows them to move in different directions to other storm cells surrounding them, which can make them difficult to predict ahead of time.

The best place to find information on active supercell thunderstorms are the severe thunderstorm warnings issued by the Bureau of Meteorology. These alerts will let you know if a supercell is capable of causing giant hail, destructive winds or tornadoes.

Supercells are possible in central western and northern inland areas of NSW today and may persist into the evening, so be sure to keep up to date with the latest warnings.

- Weatherzone

© Weatherzone 2018

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