How to make sense of the Bureau of Meteorology’s rainfall forecast.
What if we told you that you were reading the weather forecasts all wrong?
I thought I was correctly interpreting the forecast information on the Bureau of Meteorology (BOM) website, but when it comes to the information on rainfall, I was making a big mistake – and apparently it’s a mistake that’s quite common.
If you visit www.bom.gov.au, and click on the forecast for your capital city you will see two pieces of information relating to rainfall. There is the ‘possible rainfall’, which will display as 0 to 1mm or something similar, and then there is a ‘chance of any rain’ section that displays the probability of rain (for example 40 per cent).
I used to understand this to mean that there was a 40 per cent of between 0–1mm in the above example, but this is not right.
The possible rainfall figure lists the chance of any rainfall above 0.2mm for the day in question. It is not forecasting the actual amount of rainfall, just that there will be rainfall, by the smallest margin the BOM considers to be significant.
The possible rainfall figures are forecast differently again. If the possible rainfall figure is 2–10mm, there is a 50 per cent chance that there will be 2mm of rain and a 25 per cent chance of 10mm. Thus, there will always be a 50 per cent chance of the number at the lower end of the range and a 25 per cent chance of the higher number in the range.
BOM senior meteorologist Rick Threlfall told the ABC that daily rainfall forecasts were the most viewed and misunderstood part of the metropolitan forecast.
"It is a fairly complicated subject and feedback on social media has shown there is a bit of confusion about the rainfall forecast, particularly the percentage chance of any rainfall," he said.
"It is quite a complicated formula – unfortunately, rainfall is not like temperature where it is a single number," Mr Threlfall said.
"Because of the way rainfall is distributed, particularly in showers and thunderstorms, there is quite a lot more to it and you do have to delve quite a bit deeper and know a bit about interpreting the numbers."
Have you been interpreting the numbers the right way when looking at the rainfall forecast? Will this better help you plan your outdoor activities?
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