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 regarding these unusually widespread stormy conditions as they move through South Australia into Victoria today.

Some areas are already experiencing golf ball-sized hail and strong winds.

While we may not always think of thunderstorms as a threat, severe storms can be surprisingly damaging. The massive Sydney thunderstorm of 1999 produced an estimated 500,000 tonnes of hail, causing widespread damage to cars and roofs. At the time, it was the costliest natural disaster on record, overtaken only by the unprecedented 2022 floods in eastern Australia – themselves partly caused by severe thunderstorms among other weather systems.

When violent thunderstorms produce heavy rain, they can often cause flash flooding. This is because extreme rain resulting 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 electrical grid. In 2016, massive rotating supercell storms brought high winds and at least seven tornadoes to South Australia, toppling cell towers and causing a statewide power outage. Smaller thunderstorms caused major disruptions in Victoria in February this year after six towers were knocked down.

But what makes a thunderstorm ‘severe’?

The ingredients for a storm

What Causes Thunderstorms? Climate scientists and meteorologists often talk about the ingredients needed for thunderstorms.

A normal thunderstorm requires a lot of moisture in the air. Then you need vertical instability in the atmosphere, that is, relatively warm, moist air at the surface and very cold air above it. You also need a mechanism to lift the warmer surface air to a level where atmospheric instability can be removed.

For a severe thunderstorm you need all these ingredients and usually one more: vertical wind shear. This means that wind speeds and direction differ with height. For example, there may be a strong northerly wind at a low level, and a strong southerly wind at a higher level.

Vertical wind shear can make an ordinary thunderstorm much more intense in several ways. For example, wind shear can keep warm updrafts separated from cold downdrafts and rainfall, making the storm last longer.

If a thunderstorm contains large hail, produces damaging wind gusts, or has the potential to cause a tornado or flash flood, it is classified as a severe thunderstorm by the Bureau of Meteorology.

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

Forecasters can predict the chance of severe thunderstorms several days in advance by looking for moist air and wind. But predicting exactly where and when they might emerge is a huge challenge.

What’s unusual about these storms?

This week’s storms are unusually widespread, with thunderstorms possible from Kalbarri in central Western Australia through Esperance, into South Australia, into Victoria and up through New South Wales and southern Queensland.

These conditions are due to a large-scale low-pressure system moving from west to east.

Prior to the arrival of this low-pressure system, winds from the north bring down moisture and instability and prepare the system for thunderstorms. When 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 energy released by condensation of water vapor. These rising air currents can travel several kilometers high in the atmosphere and even reach the top of the troposphere, 10 to 15 kilometers high.

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

Thunderstorms, tornadoes and fire

Severe thunderstorms can also pack a hidden punch. In extreme cases they can cause tornadoes.

In August, severe thunderstorms struck northern Victoria, spawning a tornado, a destructive swirling column of air that damaged homes and farms in the high country.

This surprised many people. It is well known that Australia has tropical cyclones in the north, intense tropical storms that come from the sea, but tornadoes are not as well known.

Australia does indeed experience tornadoes – an estimated 30 to 80 per year. In 2013, a total of 69 known tornadoes caused nearly 150 injuries. Many of these tornadoes spin from supercells.

In Australia’s warmer months, many fires rage across the country. Thunderstorms can worsen fires by bringing strong, warm northerly winds, often with rapid variations in speed and direction that can increase the spread of a fire.

Firefighters and first responders fear these conditions. Australia’s deadliest bushfire was Black Saturday in 2009, which killed 173 people. One of the reasons it was so dangerous was that it happened suddenly. Intense northerly winds brought down power lines and started fires, which quickly turned into intense firestorms, including thunderstorms that formed in the fire plumes.

Will climate change lead to more severe storms?

As the world warms, more water evaporates from warm sea surfaces and remains suspended in the air as water vapor. This means that more of this ingredient is needed to fuel heavy thunderstorms and more intense rain resulting from thunderstorms.

What we don’t know for sure yet is how the prevailing air currents over Australia are changing. This can move moisture to different regions or affect other ingredients of thunderstorms, such as vertical wind shear, instability and lifting mechanisms. As circulation patterns change, we could see severe storms develop in new areas or at different times of the year.

Read more: We can’t yet say if net-breaking thunderstorms will get worse – but we shouldn’t wait to find out