Interpreting BOM forecasts

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​How would you interpret this Bureau of Meteorology (BOM) rainfall forecast?

90% chance of receiving 4 to 8 mm?

WRONG. There’s a 90% ‘Chance of any rain’, with ‘Possible rainfall’ of 4 to 8 mm.

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?’

The 'Chance of any rain' 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 above, there is a 90% chance of receiving more than 0.2 mm of rain.

The first value for 'Possible rainfall' means the location has a 50% chance of receiving at least that amount of rain. The second represents a 25% chance of receiving at least that amount. For the forecast above of '4 to 8 mm', there's a 50% chance of receiving 4 mm or more, and a 25% chance of receiving 8 mm or more.

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