30% chance of a forecast being useful.
Forecasts will probably be less chaotic in the future - Times Online
What are the chances of rain today? For most of the UK only a few isolated, patchy showers are expected, but western Scotland will be thoroughly wet. This is fairly clear, but the Met Office would like to add the chances of what the weather will bring. For today there would be a 10 per cent chance of rain for most of us, but 90 per cent chance in western Scotland.
However, there are worries that people might not understand probabilities. “Scattered showers” could become a 30 per cent likelihood of rain – but is that 30 per cent of the country, or time, or are the forecasters simply guessing?
Probabilities in the forecasts show how much uncertainty there is in the weather. Some days the weather is more chaotic than others, and meteorologists measure this by running their forecasts repeatedly, each time making small changes to the observations fed into their computers. If 30 per cent of the resulting forecasts predict rain, that means a 30 per cent chance of it happening.
So 30% means it might rain in 3 out of 10 predictions - not that it will rain for 3 out of 10 hours or 3 out of 10 places, in other words pretty useless if you are rely on the forecast to plan anything. And how to they change the "observations" - either it is raining at the Biscay station or it isn't, or was the word he was looking for "guesses"? Still it is a good job they are trying to forecast only 24 hours ahead not long term trends, isn't it....