Ensemble forecasts

It's about probability

Issued 01/11/2014

Weather forecasts in the UK are usually presented in a deterministic way, in other words they present one outcome for a given time. An example of this would be to say that tomorrow maximum temperature in the forecast location will be 15C (59F) and there will be 10mm of rain. This may well be the most likely outcome especially when looking ahead a very short time. Unfortunately the further ahead the forecast is valid for the lower the level of confidence in it being correct.

One way to help address this problem is with the use of ensembles. A forecast ensemble is generated by running a computer model (or models) several times with slightly varying starting conditions. This variance helps account for errors in the initial starting conditions because it is not possible to record every detail of the atmosphere and push into a computer model.

On the web site ensemble data from the Global Ensemble Forecast System (GEFS) model has been available for a while but we've now made it more accessible as well as adding in new features such as the 'snow row' and put it all together on one page. The page shows predicted rainfall, temperature spread and the number of model runs forecasting snow.

Like the local place or postcodes on the site the ensemble data usually updates 4 times each day. This may sound quite complicated but it can be reduced down to probability and confidence and on the page there is information about what you should be looking for. 

For more information see the Ensemble forecast section.

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