Thunderstorms can be very dangerous; however, most are fairly weak and short-lived.
The few thunderstorms that become particularly powerful and last long enough
to cause significant damage are built differently than regular storms.
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.