The Bureau of Meteorology (BOM) has updated how it is communicating the rainfall forecast on its official weather app and the change has triggered a wave of confusion.
So why the change, what do all of the new numbers mean, and how does this compare to what is used in the rest of the world?
According to BOM forecaster Jonathan How, the change has come about because the previous format was often misinterpreted.
"One of the biggest misunderstandings of the previous format was that the chance of rainfall and the accompanying rainfall amount were very often read in the wrong way," he said.
"For example, if it was a 70 per cent chance [of any rain], and the rainfall amount was 1 to 3 millimetres, that would often be misinterpreted as a 70 per cent chance of between 1 and 3mm.
"But in reality, the 70 per cent chance refers to the chance of seeing any measurable rainfall, where 'any measurable rainfall' is at least 0.2mm.
"Then the range was the 50 per cent and 25 per cent decile, which wasn't very well communicated," he added.
The old format
In order to come up with the forecast, meteorologists do a series of model runs using Australian and international weather models.
The forecaster then brings their expertise and local knowledge to appropriately weight those models.
At the end of this process there are a range of possible outcomes.
How to communicate what that wide range of possible outcomes is most likely to mean, in a succinct way for the public, is where things get even more tricky.
The old methodology used by the BOM, and still in use on its website at the time of publication, communicates a rainfall range where the first number represents a 50 per cent chance of at least that amount of rain occurring.
The second number represents a 25 per cent chance of at least that amount of rain occurring.
A good way to get your head around what these percentages mean is to picture the whole range of possible rainfall outcomes lined up from lowest to highest.
So in this example, there is a 50 per cent chance of getting 15mm or more and a 25 per cent chance of getting 20mm or more.
Using this communication method, even if the forecast pans out as the models and forecasters expect, the range communicated only covers 25 per cent of the possible outcomes.
There is still a 50 per cent chance of less rainfall than 15mm and a 25 per cent chance of more than 20mm.
To reiterate, when looking at the website forecast, half the time there is likely to be less rain than the lowest number indicated as possible rainfall.
What's changed in the app?
The new format expands the range of possible rainfall outcomes communicated to the public as well as doing away with the "chance of any rain".
This time the first number represents as 75 per cent chance of at least that amount of rainfall occurring.
This is different from the old format still in use on the website, where the first number is a 50 per cent chance of at least.
While the second number represents a 25 per cent chance of at least that amount of rainfall occurring, the same top value as in the old format.
Again, picturing the example of the Monday forecast for Melbourne on August 22:
This new range, perhaps more intuitively, covers the middle 50 per cent of possible outcomes.
So in this example, the forecast suggests there is a pretty good chance there will be somewhere around 9 to 20mm of rainfall in Melbourne on Monday.
Further down, the app now also expresses the 75, 50, and 25 per cent chance of at least values directly as three separate numbers.
"We have had that feedback that people miss the chance of rainfall. But the whole idea is that that whole chance of rainfall was often being misinterpreted," Mr How said.
"This way is hopefully giving people a better picture of what they can expect."
App and website not matching up
According to Mr How, the BOM is planning on also updating the website to use the new format, but the time frame is uncertain.
In the meantime, there is no getting around the fact that the website and app are communicating different rainfall ranges.
Mr How concedes these discrepancies could be causing confusion.
"We've done radio crosses and articles like this just to help people understand why we made the changes and why for a period of time before the website's updated there'll be this gap," he said.
"So if people are aware of what they're looking at that should be okay. On the website it's still a convoluted way of getting to the explanation, but that'll change in time once the new website comes onboard."
What do they do in the rest of the world?
Rainfall is one of the trickiest things to forecast.
Not only are there many different factors which can impact on whether it rains, for how long and how heavily, but rainfall can also be incredibly geographically hit or miss.
Coupled with the fact that it is the easiest element for the public to notice and measure, it is a minefield for weather agencies.
Many agencies around the world do not communicate daily predicted rainfall totals as part of their online seven-day forecast equivalents.
Rather opting to stick to the chance of rain and word forecasts and symbols to indicate intensity.
In the UK, the Met Office sticks to a word forecast to indicate if showers are likely, with a daily breakdown of the change of precipitation by the hour. No quantified rainfall forecast totals are put forward.
In the US, the National Weather Service shows the likelihood of precipitation for each day and night alongside a word forecast, occasionally indicating the possible rainfall amounts within the detailed forecast.
In Japan they indicate the probability of precipitation within six-hourly periods for the next two days and in daily totals for the remainder of the week.
No likely rainfall totals are given but they do indicate normal rainfall totals for that time of year.
In New Zealand they do have an indication of how much rain is likely but they approach it rather differently than how we do in Australia.
Within the next 72 hours, they have a more detailed forecasting chart which indicates the expected rainfall in mm/hour for each hourly period.
"The difficulties with this is that it provides a very definite answer in both time and intensity but the weather isn’t always so predictable," according to Lewis Ferris, communications meteorologist at the NZ Met Service.
"Another issue with this type of forecast is that when precipitation is spotty/showery it is possible the graph won't show anything but our text forecast will mention showers. This creates an inconsistency between our products."
For the forecast four days ahead the online forecast gives a "rainfall exceedance probability."
This is the chance of getting more than the set values of 1mm and 10mm using model ensemble data.
For the record, many agencies have more information hidden in the depths of their websites.
The Australian Bureau also has more detailed forecasts available both online and on the app.
Within the MetEye section of the BOM's website there are three-hourly breakdowns of the rainfall forecast at 10, 25 and 50 "per cent chance of more than", in mm.
While in the app, scrolling down also shows this three-hourly breakdown where the "per cent chance of at least" can be selected.
At the end of the day the uncertainly around forecasting rainfall is high and communicating the range of possibilities is fraught. Forecasts are only ever a guide.
