How are our weather forecasts produced?

Most of our information is from a range of weather forecast models run for example by the Met Office.

Accuracy of models is improving decade by decade, but forecasts are increasingly poor a few days ahead, and occasionally within a day or so - when the variation of wind in the atmosphere is complicated. Unfortunately these situations are often related to more severe weather (upland gales and heavy rain/snow). Therefore as forecasting severe conditions is critical to mountain safety, our forecasts are probability forecasts. We explicitly state when the forecast confidence is low.

We use a range of words in order to keep the texts 'fresh', but there are a number of commonly used words that have probabilities attached to them.

Almost nil 5% (or less)
Rare 10% (or less)
Very unlikely/occasionally 20%
Unlikely 30%
Toss-up 50%
Likely/frequent 70%
Very likely/very frequent 80%
Almost constant 90% (or more)
Almost certain 95% (or more)

We normally reserve the vaguer terms 'mostly' and 'mainly' to help describe areas, such as when there is variation across a region as to just how far say low cloud on coastal mountains spreads inland (eg, mostly coastal hills)

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