Melbourne’s wettest October day on record produced 61.4 mm of rain in 24 hours. This Thursday, the Bureau of Meteorology is forecasting a ‘possible rainfall’ range of 45 to 70 mm. So, is Melbourne about to register a new October daily rainfall record?

At first glance, it may seem like Melbourne has a good chance of receiving close to 70 mm of rain on Thursday. After all, the number is right there in the forecast.

Image: Bureau of Meteorology forecast for Melbourne on Thursday, October 13, 2022. This forecast was issued at 5:05 am AEDT on Tuesday, October 11, 2022.

However, the ‘possible rainfall’ range shown in the Bureau of Meteorology’s forecast is often misunderstood, and the likelihood of receiving the higher amount in this range is lower than many people think.

The ‘possible rainfall’ range shown in the Bureau of Meteorology’s forecasts is based on probability.

The first amount shown in the forecast, 45 mm in the Melbourne example above, represents a 50 percent chance of at least that much rain falling in one day.

The second number, 70mm in this case, represents a 25 percent chance of at least that amount of rain occurring.

The ‘chance of any rain’ refers to the likelihood of that location receiving a measurable amount of rainfall (at least 0.2 mm) at any time that day (midnight to midnight).

So in reality, when this forecast was made on Tuesday morning, there was a 95 percent chance that it would rain at all on Thursday. However, there was only a 25 percent chance that Melbourne would get 70 mm or more on Thursday, which is a one-in-four chance.

While 70 mm is certainly possible, it is more likely that the city will get closer to or even below 45 mm, given there is only a 50 percent chance of seeing that much rain or more.

If Melbourne does manage to receive more than 61.4 mm of rain on Thursday, it will be the city’s wettest October day in records dating back to 1855, beating the previous record from October 31 in 2010.