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It might seem attractive to identify the member which verifies best in the early part of the forecast (say at T+12) and assume this member will continue to provide the best forecast during the rest of the medium range period.  But this is not true; the performance of any member during the first 12hrs of the forecast has little relevance to its skill beyond T+48hrs in the same area.

Clustering

In order to compress the amount of information being produced by the ensemble and highlight the most predictable parts, individual ensemble members that are "similar" according to some measure (or norm) can be grouped together.   This process is known as clustering.  

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