Cyclone watch: What can we expect if big one comes?

WHAT can we expect if a category one to five cyclone crosses the Queensland coastline?

According to the Bureau of Meteorology, if we are hit with a severe tropical cyclone, we could face wind gusts from 90km/h to in excess of 280km/h.

Gale force winds can extend hundreds of kilometres from the centre of a cyclone and vary in speed.

While the bureau is becoming more expert at forecasting possible paths, it's hardly an exact science.

Research has shown that cyclones in the Australian region exhibit more erratic paths than cyclones in other parts of the world.

A tropical cyclone can last for a few days or up to two or three weeks. Movement in any direction is possible including sharp turns and even loops, the bureau says.

Cyclones in the Australian region exhibit more erratic paths than cyclones in other parts of the world. Source: BOM
Cyclones in the Australian region exhibit more erratic paths than cyclones in other parts of the world. Source: BOM

What is a Tropical Cyclone?

Tropical Cyclones are low pressure systems that form over warm tropical waters and have gale force winds (sustained winds of 63 km/h or greater and gusts in excess of 90 km/h) near the centre.

The gale force winds can extend hundreds of kilometres from the cyclone centre.

If the sustained winds around the centre reach 118 km/h (gusts in excess 165 km/h). then the system is called a severe tropical cyclone. These are referred to as hurricanes or typhoons in other countries.

The Bureau of Meteorology released this tracking map of a potential tropical cyclone at 5am on Wednesday.
The Bureau of Meteorology released this tracking map of a potential tropical cyclone at 5am on Wednesday. Bureau of Meteorology

Eye of the storm

The circular eye or centre of a tropical cyclone is an area characterised by light winds and often by clear skies.

Eye diameters are typically 40 km but can range from under 10 km to over 100 km.

The eye is surrounded by a dense ring of cloud about 16 km high known as the eye wall which marks the belt of strongest winds and heaviest rainfall.

So where does the power come from?

Tropical cyclones derive their power from the warm tropical oceans and do not form unless the sea-surface temperature is above 26.5°C.

Once formed, however, they can continue over lower sea-surface temperatures.

They usually dissipate over land or colder oceans.

The BOM radar showing the white eyes of Cyclone Lam in the Gulf of Carpentaria and the tropical low off the eastern coast.
The BOM radar showing the white eyes of Cyclone Lam in the Gulf of Carpentaria and the tropical low off the eastern coast.

Cyclone danger and impacts

Tropical Cyclones are dangerous because they produce destructive winds, heavy rainfall with flooding and damaging storm surges that can cause inundation of low-lying coastal areas.

Cyclones have wind gusts in excess of 90 km/h around their centres and, in the most severe cyclones, gusts can exceed 280 km/h.

These very destructive winds can cause extensive property damage and turn airborne debris into potentially lethal missiles.

It is important to remember that, during the passage of the cyclone centre or eye, there will be a temporary lull in the wind.

But this will soon be replaced by destructive winds from another direction.

Ebony Battersby

Heavy rainfall can produce extensive flooding

Authorities urge residents, particularly children, to stay well away from floodwaters.

Motorists should not attempt to cross flooded roadways.

The heavy rain can persist as the cyclone moves inland and starts to die down.

The destructive winds that come with cyclones also produce phenomenal seas, which are dangerous both for vessels out at sea and those moored in harbours.

These seas can also cause serious erosion of foreshores.

Brett Wortman

Storm surge and tides

Potentially, the most destructive phenomenon associated with tropical cyclones that make landfall is the storm surge.

Storm surge is a raised dome of water about 60 to 80 km across and typically about 2 to 5 m higher than the normal tide level.

If the surge occurs at the same time as a high tide then the area inundated can be quite extensive, particularly along low-lying coastlines.

Categories of Cyclones

Category Strongest gust (km/h) Typical effects
1 Tropical Cyclone Less than 125 km/h 
Gales
Minimal house damage. Damage to some crops, trees and caravans.Boats may drag moorings.
2 Tropical Cyclone 125 - 164 km/h
Destructive winds
Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small boats may break moorings.
3 Severe Tropical Cyclone 165 - 224 km/h 
Very destructive winds
Some roof and structural damage. Some caravans destroyed. Power failure likely.
4 Severe Tropical Cyclone 225 - 279 km/h 
Very destructive winds
Significant roofing and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.
5 Severe Tropical Cyclone More than 280 km/h 
Extremely destructive winds
Extremely dangerous with widespread destruction

Source: Bureau of Meteorology



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