Severe thunderstorms: what you need to know

A thunderstorm is a tall cloud, that we call a cumulonimbus cloud, that produces lightning, turbulence and thunder.

Thunderstorms need three ingredients to form: The first ingredient is moisture. What we mean by that is moist, humid air that carries a lot of water vapour.

The second key ingredient for thunderstorms is a lifting mechanism. That is the force that makes the air move upwards, and quite often that can be a cold front moving in: where the colder, denser air lifts the air ahead of it. It could be air simply trying to cross a mountain range. It could be a sea breeze moving inland; or it could just be the heating of the day creating strong thermals and lifting the air higher and higher.

The third ingredient is atmospheric instability. This often comes in the form of cold air aloft and warm air lower down. That means that the warm air, as it rises into the cold air above it, can freely keep rising because it stays warmer than the environment, and therefore it can keep rising and the moisture in it condenses and forms the cloud.

A typical thunderstorm lives for about half an hour to an hour, and it goes through three stages in its evolution: The first stage is the towering cumulus stage, where the entire cloud consists of updraft – that is air rising.

The second stage it the mature stage. Now the thundercloud is at its largest size, and at its most organised. Now, in addition to the updraft, there's also a downdraft – that is air descending – and most of the heavy rain occurs in the downdraft. Then the cloud now also produces thunder and lightning. And another feature of the storm cloud now is what we call an anvil: That is the cloud spreading out sideways at its top, where the air can no longer rise any further, so the only place where it can go is sideways.

The third and final stage is the dissipating stage. That is where the energy supply to the storm is coming to an end, so the updraft and its downdraft are dying, and the cloud dissolves. Most thunderstorms in Australia form during the warm season, that is in the months of September through to March, but we can also experience cool-season thunderstorms, and they can form when the lifting mechanism is particularly strong. We can broadly forecast where and when thunderstorms form, but is it much harder to impossible to forecast where exactly and when exactly thunderstorms form. This is because very small differences in temperature and moisture make the difference between a large thunderstorm event and no thunderstorm event at all.

So if thunderstorms are forecast for your area, keep an eye on the Bureau's website for the latest warnings and also keep an eye on the radar to see whether storms are headed your way.

Thunderstorms regularly occur across Australia. But what makes a thunderstorm severe? What sets severe thunderstorms apart is their higher risk of impacts to people and property.

They are classified as severe if they are producing or expected to produce at least one of four hazards, including:

Damaging winds above 90 km/h. Heavy rain leading to flash flooding. Large hail that is greater than 2 cm in diameter. And tornadoes. When these are expected or being observed, the bureau will issue a severe thunderstorm warning.

Some thunderstorms may produce higher end hazards, including destructive winds above 125 km/h. Intense rain leading to life threatening flash flooding and giant hail greater than 5 cm in diameter.

Knowing when a severe thunderstorm is approaching your location is important. You can stay up to date by checking our website, or by having notifications enabled on the BOM Weather app.