The Bureau of Meteorology (BoM) has changed the daily rain forecasts on its app following widespread confusion.
The weather bureau’s forecasts, still used on its website, gave a possible rain reading, and a percentage chance of any rain.
For example, on Wednesday, August 24, Melbourne was forecast to have anywhere between 2 to 4 millimetres of rain, with a 90 per cent chance of any rain.
Many people would interpret this to mean there was a 90 per cent chance of rain of between 2 to 4mm.
But they would be mistaken.
In fact, the first digit in the rainfall range represents a 50 per cent chance of at least that amount of rain.
The second digit represents a 25 per cent chance of at least that amount of rain.
So, in Melbourne on Wednesday, Melbourne was 50 per cent likely to have more than 2mm of rain, and 25 per cent likely to have more than 4mm of rain, with a 70 per cent chance of any rain at all.
Confusing, right?
The weather bureau acknowledged this could be confusing, so they developed a simplified layout on their app.
The ‘‘chance of any rain’’ feature is no longer on the app.
Instead, the first digit of the rainfall range means there was a 75 per cent chance of at least that amount of rain.
The second digit was now a 25 per cent chance of that amount of rain.
According to the new view, Melbourne was 75 per cent likely to have more than 0mm of rain on Wednesday, and 25 per cent likely to have more than 4mm of rain.
The app also has ‘‘Show more about today’’, a new feature that further breaks down the probability of rain.
However, that feature isn’t yet available on the bureau’s website.
If you’re still using the bureau’s website for all your weather information, then keep the 50-25 per cent rule in mind, rather than the new 75-25 rule used for the app.
The new changes have been rolled out on the weather bureau’s app, but the old layout and system remains on the bureau’s website.
Having two different rainfall forecasts produced by the same weather bureau is certainly confusing.
But BoM forecaster Jonathan How said as long as people know what to look out for when reading both the mobile and desktop versions of the forecast, all should be fine.
‘‘If people are aware of what they’re looking at that should be OK,’’ he told the ABC.
‘‘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.’’
He said he hoped the new mobile features will be more easily interpreted.
‘‘This way is hopefully giving people a better picture of what they can expect,’’ he said.
The overhaul comes after the bureau announced last Tuesday that it was moving to a La Nina ‘‘alert’’.
That means there’s a 70 per cent chance that a rare third consecutive event will form by the end of the year.
And with the odds in La Nina’s favour, it is probable we can expect more rain.
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