Severe Thunderstorms: What You Need To Know

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.

When we see or forecast these 'severe' thunderstorms, the Bureau issues warnings so people can keep themselves, their families and their property safe. All thunderstorms need three basic ingredients and they are: Warm, moist air at the surface; a lifting mechanism, forcing air upwards; and instability. Instability is where warm, moist air at the surface rises into the atmosphere above. In a typical thunderstorm the updraft transports the warm, moist air up through the storm. Now as the air cools and condenses, clouds and precipitation form; and once this precipitation becomes too heavy gravity takes over and pulls the rain and hail down to the surface with a cool gust of wind; and it's this cool air that acts to kind of interfere with the warm updraft, decaying the storm and leading to weakening.

For severe thunderstorms to form we need these three ingredients to be strong, but also a fourth ingredient, which is wind shear. Wind shear is the increasing speed with height, as well as a change in direction, as you move up through the atmosphere. The reason that wind shear is important is because it leads to tilting and rotation of the updraft into a kind of sloping corkscrew structure. This organisation means the cool downdraft is separated from the warm updraft, so you don't see any of that interference and weakening that we see in your garden-variety thunderstorms. This means that they can last many hours and travel long distances, potentially causing considerable damage.

So when the Bureau observes or forecasts conditions that support the formation of a severe thunderstorm - such as deep rotation, separation, as well as other important features - we issue a Severe Thunderstorm Warning.

The first thing we're concerned with is heavy rainfall, which may lead to flash flooding, and this is when the rate of rain is so high that water accumulates rapidly on the ground - faster than it can run off into drains and waterways. Rain like this has a potential to cause landslides, destroy homes and even carry away cars. The Bureau also warns for large or giant hail - greater than 2 cm in diameter - but we can get much larger than that.

When you get a highly organised severe thunderstorm, we can see cricket- to even softball sized hail - up to 10 cm in diameter. That can shred trees, damage cars, roofs and even houses. You definitely don't want to be outside in that. The third impact we warn for is damaging winds. A damaging wind gust is a gust in excess of 90 km/h, which can cause falling trees and other debris to damage houses, cars and crops. And also tornadoes: The Bureau will include tornadoes on a warning if we receive a verified tornado observation. While not as numerous or powerful as those seen overseas, Australia will see around 50 tornadoes every year and they can cause extremely strong winds in a localised area, which can cause major structural and property damage.

Lightning is another dangerous element of thunderstorms, but it occurs with all thunderstorms and not just severe ones. Sometimes regular storms can create severe weather, particularly when they form close together or line up; however, the most dangerous type of thunderstorm is a supercell thunderstorm.

Supercell thunderstorms are highly organised thunderstorms: They have a very deep rotating column of air, which tilts away from the downdraft. This means they can last a long time: Four to even eight times longer than your regular thunderstorms.

The peak season for severe thunderstorms in Australia is from around September through to March or April, although we can see severe thunderstorms in the winter months as well - these are mostly connected to strong cold fronts across southern parts of Australia.

So whenever severe thunderstorms affect your area, keep an eye on the satellite or radar to see if storms are headed your way and always get the latest forecasts and warnings via the Bureau's website or the BOM Weather app.