Knowing the different types of weather warnings from the Bureau of Meteorology and the triggers for those warnings can help you better understand thunderstorms in your area.
Is it a robot or a human issuing the warning?
While details like location, mapped warning areas and time-stamps are automated, all of the information in the warning is edited and reviewed by a real, live BOM meteorologist before being issued to the public.
Some organisations like the ABC are also responsible for relaying the warning information to the public, and we also play a role in translating some of the technical jargon when we broadcast the warning during regular weather segments or special emergency updates.
What are the different types of BOM weather warnings?
Severe Weather Warnings — issued to warn people about potentially hazardous or dangerous weather. Severe weather is different to severe thunderstorms, tropical cyclones or risky bushfire weather and can include:
- Damaging wind — wind gusts greater than 90 km/h or average winds speeds greater than 63 km/h.
- Heavy rainfall — rainfall over a period of time which is greater than the 10 per cent Annual Exceedence Probability (AEP), i.e, the likelihood that it will rain more than a certain amount in a certain period of time. If you want to get technical, you can read about the AEP in our glossary of terms.
- Blizzards — issued for NSW and Victoria, for elevations higher than 1200 m.
Severe Thunderstorm Warnings — for potentially hazardous or dangerous weather directly related to thunderstorms (a storm with lightning). Lightning can start bushfires, destroy trees and knock out electrical and telecommunications towers.
Coastal Hazard Warnings — for abnormally high tides or storm tides. Coastal hazards include erosion, huge waves and inundation.
Advice on everyday weather — Marine Wind Warnings; Frost Warnings; agricultural warnings such as Sheep Graziers' Warnings, Brown Rot Advice, Downy Mildew Advice and Bushwalkers' Alert (Tasmania only).
Other warnings — Heatwaves, Tropical Cyclones, Fire Weather, Floods, Tsunamis, Damaging and Dangerous Surf, High Seas and Geomagnetic Storms.
What is the trigger for a Severe Thunderstorm Warning?
The BOM issues Severe Thunderstorm Warnings when forecasters detect or expect intense and potentially dangerous thunderstorms with:
- large hail — 2 cm in diameter or larger
- damaging wind gusts — 90 km/h or greater
- heavy rainfall — rainfall over a period of time which is greater than the 10% Annual Exceedence Probability or a one-in-10 chance of rain this heavy
- a tornado.
Why is the word "dangerous" sometimes used in a storm warning?
The BOM classes a storm as "high-end" or "dangerous" if it produces:
Destructive wind gusts — gusts greater than 125 km/h. Winds like this can topple trees and send trampolines flying.
Giant hailstones — greater than 5 cm in diameter. Falling lumps of ice the size of an egg can really do some damage to your noggin and your car.
Intense rainfall — rainfall over a period time which exceeds the 2% AEP, or, a one-in-50 chance of rain this heavy.
Tornado — a violently rotating column of air over land with an intense updraught near the centre. A tornado has enough strength to rip the roof off your house, or send a small dog flying.
For NSW, if the rainfall is greater than 70 mm in (up to) an hour or greater than 150 mm in (up to) three hours it is considered intense.
Where does the BOM gather information from?
The Bureau's forecasts and warnings are based on data from a radar and observation network of more than 11,000 pieces of equipment. This includes equipment on the land, in the sea and in the sky and space such as radars, automatic weather stations, flood gauges, upper atmosphere monitoring devices and ocean wave buoys.
Why are some forecast areas really big but only a small area ends up being hit by the thunderstorm?
The Bureau can analyse and then predict when the atmosphere is likely to produce severe thunderstorms but can't pinpoint or guarantee the exact location of individual storms ahead of time because:
Thunderstorms are complex. They can develop quickly, move fast, and are strongly influenced by environmental factors like temperature and moisture changes, sea breezes and topographical effects.
Storms can be small-scale — storms can be as small as a few hundred metres in diameter, making it hard for computer modelling, which generally deals with data on the scale of 10 kilometres or more, to capture.
As storms develop within a favourable environment, the Bureau's teams start to track them.
When they approach a major population centre, they'll issue Detailed Severe Thunderstorm Warnings to track separate storm cells. These warnings appear with a red dot where each of the storm cells are and arrows to show which direction the storm is moving.
The most dangerous weather associated with thunderstorms usually happens directly beneath the storm itself. This means that if the storm's path varies by as little as 200 metres, it could mean the difference between a lot of rain where you are, or not much rain at all.
This is why you should act when thunderstorms are forecast for your area — move cars undercover, secure loose outdoor items and unplug electrical devices. This way you'll be ready whether you get a lot of rain and storm damage or not.
How does the ABC deal with the different BOM warnings?
Your local ABC radio presenters will mention things like Severe Weather Warnings and Sheep Grazier Warnings as part of their regular weather segments.
The ABC Emergency team monitors all warnings from the BOM and works with local ABC radio teams to keep audiences across any other type of weather that could threaten lives or properties, via radio, social media and News Online articles. This includes warnings for severe weather, thunderstorm forecasts, floods, tropical cyclones and heatwaves.
Basically, if weather is likely to impact you, you will see it on your local ABC Facebook page, ABC News Online or hear it on your radio. Remember to download the BOM Weather App to stay across all the weather and warnings for your area.