Severe thunderstorms hit South Australia. But what makes a thunderstorm “severe”?

Clusters of severe thunderstorms are expected to hit southern regions of Australia on Thursday and Friday.

The Bureau of Meteorology has issued severe weather warnings and forecasts in relation to these unusually widespread storms moving through South Australia and into Victoria today.

Golf ball-sized hail and strong winds have already occurred in some areas.

Even though we don't always think of thunderstorms as a threat, severe storms can cause surprising damage. The massive Sydney thunderstorm in 1999 dropped an estimated 500,000 tonnes of hail and caused extensive damage to cars and roofs. At the time, it was the costliest natural disaster on record, surpassed only by the unprecedented 2022 floods in eastern Australia – which themselves were partly caused by severe thunderstorms, among other weather conditions.

When severe thunderstorms bring torrential rain, they can often trigger flash floods. This is because extreme rain from thunderstorms usually falls in a relatively short period of time – in many cases less than an hour or two. Lightning can also pose a threat.

In recent years, severe thunderstorms have also shown that they can damage the power grid. In 2016, huge rotating supercell storms brought strong winds and at least seven tornadoes to South Australia, toppling cell towers and causing a nationwide blackout. Minor thunderstorms led to major outages in Victoria in February this year after six towers were destroyed.

But what makes a thunderstorm “severe”?

The ingredients for a storm

What causes thunderstorms? Climate scientists and meteorologists often talk about the necessary ingredients for thunderstorms.

To trigger a normal thunderstorm, the air must contain a lot of moisture. Then you need vertical instability in the atmosphere, i.e. relatively warm, moist air near the surface and very cold air above it. They also need a mechanism to raise the warmer surface air to a level where the atmospheric instability can be resolved.

A severe thunderstorm requires all of these ingredients and usually one more: vertical wind shear. This means that wind speed and direction vary with altitude. For example, there could be strong northerly winds in the lowlands and strong southerly winds in the higher elevations.

Vertical wind shear can make an ordinary thunderstorm much more intense in a number of ways. For example, wind shear can help keep warm updrafts separated from cold downdrafts and precipitation, which can help the storm last longer.

If a thunderstorm has large hail that causes damaging wind gusts or could trigger a tornado or flash flood, it is a severe thunderstorm, according to the Bureau of Meteorology classification.

You may also have heard of supercell storms. These are convective thunderstorms characterized by strong, rotating updrafts that last for a long time.

Meteorologists can predict the potential for severe thunderstorms several days later by paying attention to moisture-laden air and winds. However, it is extremely difficult to predict exactly where and when they might appear.

What's unusual about these storms?

This week's storms are unusually widespread. Thunderstorms are possible from Kalbarri in central Western Australia down to Esperance, further into South Australia, into Victoria and up into New South Wales and southern Queensland.

These conditions are due to a large area of ​​low pressure moving from west to east.

Before this low pressure system arrives, northerly winds bring moisture and instability, preparing the system for thunderstorms. As the air near the low pressure system begins to rise, energy can be released from the warm, moisture-laden and unstable air. This also includes the release of energy through condensation of water vapor. These rising air currents can penetrate several kilometers into the atmosphere and even reach the top of the troposphere at an altitude of 10-15 km.

Severe thunderstorms in South Australia are more likely in spring and summer. That's because there is plenty of moisture in the tropics and warm oceans around Australia, while low pressure systems and cold fronts can still form in the cold oceans to our south.

Thunderstorms, tornadoes and fires

Severe thunderstorms can also have hidden effects. In extreme cases they can trigger tornadoes.

In August, severe thunderstorms hit northern Victoria, triggering a tornado, a destructive, swirling column of air that damaged homes and farms in the highlands.

This surprised many people. It is well known that there are tropical cyclones in the north of Australia, i.e.

In fact, there are tornadoes in Australia – an estimated 30-80 per year. In 2013, a total of 69 known tornadoes caused nearly 150 injuries. Many of these tornadoes arise from supercells.

During Australia's hotter months, many fires burn across the country. Thunderstorms can worsen fires by bringing strong, warm northerly winds, often with rapid changes in speed and direction, which can increase the rate at which a fire can spread.

Firefighters and first responders fear these conditions. Australia's deadliest bushfire was Black Saturday in 2009, which killed 173 people. One reason it was so dangerous was its suddenness. Strong northerly winds brought down power lines and sparked fires that quickly grew into intense firestorms, including thunderstorms in the fire plumes.

Will climate change bring stronger storms?

As the world warms, more water evaporates from warm ocean surfaces and hangs in the air as water vapor. This means more of this ingredient is needed to produce severe thunderstorms and more intense rain from thunderstorms.

What we don't yet know exactly is how the prevailing air currents over Australia are changing. This could shift moisture to other regions or affect other thunderstorm factors such as vertical wind shear, instability and uplift mechanisms. If circulation patterns actually change, severe storms could occur in new areas or at different times of the year.

Read more: We can't tell yet if the grid-busting thunderstorms are getting worse – but we shouldn't wait to find out