Ensemble mean and spread: from the ECMWF ‘Ensemble Prediction System’ (ENS), for four parameters: mean sea level pressure, 850 hPa temperature, 850 hPa wind speed and 500 hPa geopotential height.

These charts are updated once every 12 hours at approximately 08:30 UTC and 20:30 UTC. Each chart header is labelled with the date and time when the ensemble forecasts were initiated (D0), which will be 00UTC for the 08:30 UTC update, and 12UTC for the 20:30 UTC update. Each map is then valid for a date between D0 + 1 and D0 + 10days, which is indicated in the chart header by VT (=Valid Time) and which can be adjusted using drop down menus above the plot (grey boxes). Additional drop down menus here enable the user to change parameter and geographical region.

One ‘ensemble forecast’ consists of 51 separate forecasts made by the same computer model, all activated from the same starting time. The starting conditions for each member of the ensemble are slightly different, and physical parameter values used also differ slightly. The differences between these ensemble members tend to grow as the forecasts progress, that is as the forecast lead time increases.

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