Why you're reading forecasts wrong

Bureau of Meterology forecasts: Why you're reading them wrong

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Daily forecasts, like the one on the right, are often misinterpreted.

Daily forecasts, like the one on the right, are often misinterpreted.

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Reading forecasts isn't what it seems.

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Producers frequently check rainfall forecasts and often find themselves disappointed when their expectations don't become reality.

But years after information about forecasting changes was released to the public, the Bureau of Meteorology's daily town forecasts are still not being interpreted correctly.

Daily rainfall forecasts from BOM offer details on the chance of rain and possible rainfall measurements.

Often people will correlate the percentage for chance of rain with the possible rainfall amount but that isn't correct.

An example of a daily forecast. Photo: BOM

An example of a daily forecast. Photo: BOM

Forecast figures explained

The chance of rain figure refers to the likelihood of any rainfall in a given location.

In the example above, there is a 70 per cent chance of getting rain in the Sydney location. It shouldn't be interpreted as a 70 per cent chance of receiving four to 15 millimetres.

Instead, the first number of possible rainfall represents a 50 per cent chance of at least that figure falling while the second number has a 25 per cent chance.

Using the same example, there is a 50 per cent chance of at least 4mm falling and a 25 per cent chance of up to 15mm.

"To simplify things, it's worth breaking these fields down into two questions: 'Will it rain today?' and 'If it does, what sort of rainfall amount is likely?'" a BOM statement said.

"Days where the rainfall range is larger generally suggest a storm day where anyone under the storm path will receive a lot of rain but others may stay completely dry or get just a few drops.

"On the other hand when the rainfall range shows reasonably large totals but a relatively small range like 15 to 20 mm, there's usually widespread rain falling from a broad-scale cloud system, not just showers or thunderstorms popping up here or there."

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The story Why you're reading forecasts wrong first appeared on The Land.

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