Right as rain: How to interpret the daily rainfall forecast
20 May 2014
They’re probably the single most viewed components of the Bureau’s town forecasts. But that doesn’t mean it’s not easy to misconstrue the rainfall figures…
The Bureau’s daily rainfall forecasts help us plan our day every time we step out of the door. But what do they actually mean? Looking at one of the latest forecasts for Melbourne, we are told there’s a 60% “Chance of any rain”, with “Possible rainfall” of 2 to 5 mm. But these two fundamental facts should not be run together: they do not mean there is a 60% chance of receiving between 2 and 5 mm of rain.
To simplify your interpretation, it’s worth breaking these two fields down into two basic questions: “Will it rain today?” and “How much rain will we get if it does?”
The first question is answered by the “Chance of any rain”. Technically this value refers to the likelihood of receiving more than the minimum measurable amount of rainfall (0.2 mm) over the 24 hours from midnight to midnight. For the forecast we’re currently looking at, there is a 60% chance of receiving more than 0.2 mm of rain. That’s enough to pack the brolly!
The second question is answered by the “Possible rainfall”. Whenever significant rainfall is forecast, this field is represented as a range between two values (otherwise it will appear as “0 mm”). The first value represents a 50% chance of receiving at least that amount. The second represents a 25% chance of receiving at least that amount. So for the current Melbourne forecast of “2 to 5 mm”, Melbourne has a 50% chance of getting 2 mm or more, and a 25% chance of getting 5 mm or more.
Days where the rainfall range is larger generally suggest a storm day where anyone under the storm path will receive a lot of rain – while those next to the path may only get a little drizzle. During the tropical monsoon rain season it won’t be uncommon to see a rainfall range with both numbers exceeding 100 mm.
On the other hand when the rainfall range shows reasonably large totals but a relatively small range like 15 to 20 mm, this most often represents a situation when there is widespread rain falling from a broad-scale cloud system, not just showers or thunderstorms popping up here or there.
Even when there’s very little likelihood of rain, the Bureau will still provide a “Chance of rain” value to enable people to develop their own thresholds for rain-dependent decisions.
Take this example, from a cloudy day with a negligible chance of rain. The zero rainfall amount “0 mm” indicates that the Bureau’s forecasters expect no rain today – although there does remain a very low chance of rain.
So there you have it. When you leave the house in the morning, it’s worth checking the “Chance of rain” to work out if you’ll need your brolly, and the “Possible rainfall” to see what sort of shoes to wear.
A fuller picture
Numbers don’t always tell the whole story, of course. For people seeking a fuller picture of the day’s weather, the town forecasts also include a more detailed text description – with details such as whether it will be sunny or cloudy, the chance of thunderstorms or hail, and the time of day the weather is expected to change.
These additional details are ideal for people who are planning outdoor activities, or whose work or travel may be dependent upon the weather. But if your schedule will be strongly dictated by the weather, we also advise that you visit our MetEye weather map, where you can see how close you are to the area of significant rain. Sometimes a small change in the pattern can have a huge impact on how much rainfall you receive.