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To ensure synoptic consistency, each individual ensemble member must belong to the same cluster throughout the lead-time window.   For two members to be assigned to the same cluster, they must display similar synoptic 500hPa development over the whole time window.  However, weak gradients in 500hPa forecasts can lead to synoptically rather different members being assigned to the same cluster because of the the RMS norm.  The clustering scheme is designed to create no more than six clusters.

Most representative member

A cluster is represented not by the mean of its members but by its most representative member (MRM or cluster scenario).  This is selected by a pattern-matching algorithm minimising algorithm minimising root mean square differences between the cluster’’s ““centre of gravity”” “centre of gravity” of the cluster and each member.  The most representative member is chosen to symbolize the cluster.  But it should not be used as a deterministic forecast, nor should it be seen as a substitute for the cluster average (cluster averages are not currently available as web charts but are available for download from MARS).

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• If the ensemble members fall into a few, well separated groups of ““similar forecasts”” ““similar forecasts”” (a multi-modal distribution”distribution”) the most representative members (cluster scenarios) will represent the range of possible weather conditions (see Fig8.1.3.1-2).  
• If the spread of all the ensemble members is broad and represent a continuum which does not divide up logically into groups, then the cluster algorithm cannot partition the ensemble into significant different clusters and the ensemble median is presented as the most representative member (cluster scenario) of the single "cluster". 
• If the spread of all the ensemble members is small, more than one cluster can still be created provided that a partition is possible.  

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