Ok. We admit we’ve been doing it wrong.
But thanks to a recent article by the ABC we’re now enlightened.
Rainfall forecasts are often misunderstood (and if you’re anything like us easy to criticise when wrong). So, since we’ve been praying for rain all season, it might be best if we properly understood what the poor weatherman has to say.
To illustrate, look to your local weather app or BOM website and check what’s happening (or see the example below). Most forecasts will say something like “80% chance of rain, possible rainfall of 2 – 20mm”. Pre-enlightenment we saw this and thought, ‘Right, so we have an 80% chance of getting between 2 and 5mm of rain.’ Would’ve you thought the same? If yes, we’d both be wrong. Although we wouldn’t have been alone in thinking that.
To read the forecast properly, you need to separate the two parts. The first part of ‘chance of any rain’ means just that. That there is an 80% chance of any rain. ‘Any rain’ can be as little as 0.2mm (which is the smallest amount of rain that can be measured over a 24-hour period). So there’s 80% chance of getting at least 0.2mm.
The next part is a bit more complicated. Possible rainfall is the amount of rain we could get. The first number, in our example the ‘2’ will mean there is a 50% chance of receiving at least 2mm of rain. And the second number, the ’20’, means there is a 25% chance of receiving 5mm of rain.
It’s quite a complicated formula. And unfortunately, none of us here have degrees in meteorology to explain it better. So, for now, we’ll trust what the experts have to say.
At least by knowing this, our expectations might be more realistic (and less likely to be disappointed!).
Original article: www.abc.net.au/news/2018-08-18/rainfall-forecasts-misunderstood-how-to-make-sense-of-them/10093822