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The 500hPa height plume shows most ensemble members show heights remain above 578dam until Wednesday 28 June before becoming more uncertain later.  Note a few ensemble members fall below 570dam on Monday/Tuesday 26/27 June suggesting a possible though less likely passage of an upper trough.

In contrast to Meteograms, plumes can display bi-modal characteristics.  Large-scale bi-modality shows uncertainty in the ensemble evolution (e.g. part of the ensemble may favour a transition to blocking; the rest may favour a zonal regime).  Local bi-modality reflects smaller scale location or timing uncertainties (e.g. a front or minor low is forecast by different members either upstream or downstream of a particular location, resulting in quite different local weather forecasts).  It is important to distinguish between these kinds of bi-modality.  Large-scale bi-modaility might for example be denoted by a bi-modal 500mb height plume (perhaps accompanied by a bi-modal 850mb temperature distribution).  Local front-related bi-modality might be indicated by having at the same time a bi-modal 850mb temperature plume and a 500mb height plume that is not bi-modal.  Clustering products can also help in the detection of large-scale bi-modality.

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