Ensemble and probability forecasting

Many people, at one time or another, have been caught out by an inaccurate weather forecast, but why do forecasters sometimes get the weather wrong and what can we do about it? This section of our web site looks at understanding the uncertainty in weather prediction and how the risks of particular weather events can be estimated using an ensemble forecasting system.

What is an ensemble forecast?

An ensemble forecasting system samples the uncertainty inherent in weather prediction to provide more information about possible future weather conditions. Rather than producing a single forecast, multiple forecasts are produced by making small alterations to either the starting conditions or the forecast model, or both.

Find out more about ensemble forecasts

The Met Office Short-range Ensemble System — MOGREPS

One of the ensemble forecasting systems we use is the Met Office Global and Regional Ensemble Prediction System (MOGREPS). MOGREPS produces ensemble forecasts for one to two days ahead. Each MOGREPS ensemble forecast is a set of 24 separate model predictions for the North Atlantic and Europe, and the range of solutions among the predictions provides a quantitative measure of the uncertainty associated with the forecast.

Find out more about MOGREPS

Medium-range ensemble forecasting

Forecast uncertainty grows rapidly as we try to predict the weather further ahead, so ensemble forecasts that focus on the uncertainty in medium-range forecasts (three–15 days) have been in use for much longer than the short-range ensemble forecasts. We are currently using MOGREPS to produce experimental medium-range forecasts, but our operational medium-range ensemble forecasts are provided by the European Centre for Medium-range Weather Forecasts (ECMWF).

Find out more about medium-range ensemble forecasting

Probability and decision-making

The output from the ensemble systems allow the uncertainty of the forecast to be quantified, and the risk of a particular weather event occurring can be assessed. This can aid decision-making for those who are sensitive to the occurrence of certain weather events.

Find out more about decision-making

Positive impact of ensemble forecasts

By assessing the uncertainty in the forecast, ensemble forecasting can help decision-makers judge the appropriate level of response to a risk of high-impact weather. Where the predicted risk of a severe weather event is high this can give forecasters the confidence to issue earlier warnings. If there is a smaller probability of a severe event then emergency planners and vulnerable businesses can be alerted to protect their vulnerable assets. Some examples can be found on the positive impacts page.