This is how the Bureau of Meteorology’s rain forecast is interpreted correctly

With pandemic restrictions allowing larger groups of people to socialize outside, it has become more important than ever for many in COVID hotspots if and when rain is predicted.

If you have been trying to plan a picnic for the past few weeks, you may feel that the weather forecast has outlived the rain but then subdued.

But is the forecast inaccurate? Or are you reading it wrong?

We spoke to senior meteorologist Diana Eadie, head of forecasting services at the Bureau of Meteorology in Victoria, to find out.

How can I tell if it’s going to rain at a particular time and place?

Read it in small print! Just because the icon looks rainy does not mean it will rain all day or at your specific location.

In fact, there could be no rain at all.

For a more accurate prediction, Ms. Eadie recommends looking at the font next to the icon and the top temperature.

Spectators watching the Melbourne Cup parade stand in the rain holding umbrellas.
In 2019, Melbourne Cup parade spectators came prepared with umbrellas and raincoats to catch a glimpse of race royals.(ABC News: Dylan Anderson)

The app or the corresponding online Met Eye are also useful.

Specifying your location will give you the forecast for a 3-kilometer square area around you.

You can also see a weather forecast hour by hour where the agency will tell you the probability of rain and how much rain can fall each hour.

What does the percentage chance of rain mean?

There are two parts to the agency’s rain forecast: the chance of rain and the amount of possible rainfall.

Beware – they must both be read completely separately!

“I think it gets misinterpreted at times,” Mrs Eadie said.

Ms Eadie says the reason the agency uses 0.2 mm is that it is the lowest amount of precipitation that can be measured.

So if the forecast says that there is a 60 percent chance of rain, it means that there is a 60 percent chance that at least 0.2 mm of rain will fall in your area.

It also means that there is a 40 percent chance of no rain – even if the icon shows raindrops.

Inside a Doppler radar
Inside a Doppler radar is a rotating dish that emits electromagnetic pulses. When the pulses hit raindrops, they are returned, which tells the forecasters how much moisture is falling and where. (ABC News: Mark Bennett)

What about possible rainfall?

The second part of the forecast is the amount of rain that may fall.

“When measurable precipitation is predicted, this field is represented as an interval between two values [otherwise it will appear as 0mm]”, says the agency.

“The first value means that the site has a 50 percent chance of receiving at least that amount of rain.

“The other represents a 25 percent chance of receiving at least that amount.”

A forecast for the Melbourne area shows an 80 per cent chance of rain
An example of a Melbourne weather forecast from the Bureau of Meteorology.(Delivered: BOM)

In this example, there is not an 80 per cent chance that the Melbourne area will get 2-4 mm of rain.

There is an 80 percent chance that the Melbourne area will receive 0.2 mm of rain.

If it rains, there is a 50 percent chance that Melbourne will receive at least 2 mm of rain and a 25 percent chance that the city will receive at least 4 mm.

“So if it says that there is an 80 percent chance of rain, but it will only be 0-1 mm, it probably will not have [as much of] an impact on your plans as a high amount of rainfall, “Ms Eadie said.

How does the agency arrive at the weather forecast?

The Bureau of Meteorology obtains most of its data from hundreds of automated weather stations across the country, radars and satellite imagery.

A satellite image of Australia with cloud over the bite, Victoria and much of Queensland
The agency’s meteorologists use high-quality satellite imagery to help them predict the weather.(Delivered: BOM)

The Himawari satellite sends images back every 10 minutes – giving meteorologists information about storms and cyclones that form, how low-pressure systems move through and even smoke rising from significant fires.

You can even see the satellite on the agency’s website.

    three pieces of technology that form a weather station: a high radar and two boxes on the ground.
The agency uses data collected from hundreds of automated weather stations like this one.(Delivered: BOM)

Mrs Eadie said all this data – on temperature, humidity, wind and more – was then entered into a supercomputer.

“It helps us project what the weather systems will do in the future.”

Our weather model is called the Australian Community Climate and Earth-System Stimulator, or ACCESS.

Ms Eadie said progress in information quality had allowed the agency to consistently improve the accuracy of its forecasts.

How accurate is the forecast?

Predicting future weather is inherently an inaccurate science, Ms Eadie said.

“The models we use have an inherent uncertainty – in general, the shorter the time frame, the fewer errors,” she said.

The agency’s forecasters are dependent on Australia’s ACCESS weather model, but they also look at the weather models from Japan, Europe and America.

A young boy is standing in a puddle in the yard at sunset
Puddles dry out in WA, with spring set to bring sunshine.(Delivered by: Jackie Grylls)

If all models agree, then the agency has more confidence in its forecast than if the models all say different things.

The types of weather systems in the area can also change how safe the agency is in the weather forecast.

Ms Eadie said low-pressure systems and cold fronts (which can lead to sudden weather changes and storm activity) are harder to predict than high-pressure systems (which tend to lead to calmer weather).

During coronavirus lockdowns and border closures, there has been some speculation that having fewer planes in the sky has affected the accuracy of weather forecasts.

Mrs Eadie said that had not been the case in Australia because weather observations from aircraft were not a primary source of data for the agency.

“From our perspective, it has not changed how we predict, or [created] a change in the quality of the data we receive, “she said.

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