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Two similar-looking forecast charts may display large differences in geopotential if they contain systems with strong gradients that are slightly out of phase.  Conversely, two synoptically rather different forecast charts will display small differences if the gradients are weak.  The spread refers to the uncertainty of the values of mean sea level pressure, geopotential height, wind or temperature, but not necessarily to the flow patterns.  Also “jumpy”, deterministic forecasts might indicate that very different weather developments are possible.   Such aspects are reflected in charts of ensemble spread.   

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Fig8.1.2.1: The diagram is a schematic plume showing the relation between the standard deviation of the ensemble members for the whole forecast range (shaded area), an individual forecast (green line), the ensemble control member (blue line) and the ensemble mean (red line).  The ensemble mean lies more or less in the middle of the ensemble spread whereas any individual ensemble member (green line), can lie anywhere within the spread.  The ensemble control, which does not constitute a part of the plume, can even on rare occasions (theoretically on average 4% of the time) be outside the standard deviation plume.

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Special composite charts have been created to allow comparisons between the ensemble mean and the ensemble control (e.g. on ecCharts) (Fig8.1.2.3).  The coloured areas do not indicate the probability of the location of a feature, but merely indicate the magnitude of the uncertainty.   Users should refer to Postage Stamp charts (example chart), Spaghetti Plot charts, or Clustering (example chart) to assess probability of departure from the ensemble mean before making forecast decisions.  

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 Fig 8.1.2.4: An example of forecast mean sea level pressure (taken from part of an ECMWF mean and spread chart) highlighting the difference between the ensemble control (green) and the ensemble mean (black).  Absolute spread of ensemble members is shown by shading.  The ensemble mean is the average over all ensemble members.  It smooths the flow more in areas of large uncertainty (large spread).  If there is large spread, the ensemble mean can be a rather weak pattern and may not represent any of the possible states.  The ensemble mean should always be used together with the spread to capture this uncertainty.  Note in particular the small depressions forecast by the ensemble control near 35W (shown by arrows) and the additional uncertainty (darker purple) within the ensemble members nearby.  This suggests at least some of the ensemble members show something similar to the ensemble control although with timing and/or location differences. 

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The normalised spread shows the increase or decrease in spread at a location.  It does not show the magnitude of the spread.  Therefore it highlights relatively low or relatively high uncertainty, but not the uncertainty itself.

ECMWF produces Mean and Spread charts and Normalised Standard Deviation charts for each ensemble run to aid understanding of the uncertainty of the forecast and whether the forecast is more or less uncertain in a given area at a given lead-time.

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