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Probabilistic terminology in the weather forecast

In certain situations, MeteoSwiss makes statements about probability in its weather forecasts. This type of information is normally given for parameters for which it is not yet possible to provide an accurate and detailed forecast. This applies in particular to forecasting precipitation, thunderstorms and cloud cover. The random nature of these phenomena makes precise and detailed forecasting difficult. However, it is possible to provide information on the probability of their occurrence and, for some of these phenomena, their geographical distribution.

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From “"not impossible” to “very likely”

The weather forecasts do not use probability percentages but, rather, specific terms that apply to a defined range. The correspondence between the probability ranges and descriptive terms is as follows:

Probability

Term

0 to 20%

Not excluded or no specific indication

20 to 40%

Possible

40 to 70%

Likely

70 to 100%

Very likely or no specific indication

A probability applies to a defined region (e.g. eastern Switzerland), i.e. the likelihood of occurrence is representative of the entire region and does not necessarily indicate the probability for a particular location.

Probabilities are sometimes accompanied by a term describing the geographical distribution of the phenomenon. This type of information is generally used for parameters of a random nature, such as sunshine, cloudor precipitation.

Examples of the use of probabilistic terminology and supplementary geographical information in the weather forecast

Weather forecast

Explanation

Tomorrow, Tuesday: Partly sunny, with showers possible in the morning. Widespread showers in the afternoon and evening; localised thunderstorms likely.

Between 20% and 40% likelihood of showers in the morning and more than 70% likelihood of showers in the afternoon and evening. 40% to 70% likelihood of thunderstorms.

Tomorrow, Sunday often cloudy, with some showers possible in the afternoon. Bright intervals likely in the Swiss plateau.

Between 20% and 40% likelihood of showers in the afternoon, affecting less than 40% of the forecast region. Between 40% and 70% likelihood of bright intervals in most of the Swiss plateau.

Tomorrow, Friday very cloudy, with bright intervals not excluded in the Lake Constance area.

Less than 20% likelihood of brief bright intervals.

Very cloudy on Wednesday with extended spells of precipitation. Snowfall limit around 700 metres, with snow falling even in low-lying parts of the inner Alps.

Over 70% likelihood of showers in most regions. Between 20% and 40% likelihood of snow falling even in low-lying parts of the inner Alps.

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