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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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The 850hPa temperature plume becomes wider beyond Sunday 25 June as the forecasts of ensemble members diverge.   Most ensemble members show a clear fall in temperature with darker shading.  A few, including Ensemble Control Forecast (ex-HRES) forecast, maintain higher temperatures rather longer.

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Fig8.1.6-2 A plume diagram for Cluj-Napoca, Romania  based on medium range ensemble run DT 00UTC 21 June 2023.  An example of tri-modal characteristics within a plume.  On Tuesday 21 June the ensemble forecasts of 850hPa temperature splits into three main branches which persist for about two days: one rising to 17°C, the other with slightly more ensemble members falling to 7°C.  The branch lying between them should be viewed with some caution as for a period there is little consistency between the evolution of members.  The forecast 500hPa heights show a split with some showing a temporary significant dip in contour heights while others show minor rise.  Note also that on 30 June, Ensemble Control Control Forecast (ex-HRES) forecast (yellow) shows greater 500hPa geopotential height than almost any member of the ensemble.

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Fig8.1.6-4: Two plumes from ensemble for Southampton, Data Time 00UTC 18 January 2017 (left) and Data Time 00UTC 19 January 2017 (right).  Upper plumes show 850hPa temperature variations among ensemble, lower plumes show 500hPa geopotential height variations among ensemble for each of the two forecasts.  The ensemble mean (not shown) lies in the spread.  Any individual ensemble member will lie anywhere within the spread, but the unperturbed ensemble control Ensemble Control Forecast is not used in the calculation.  Ensemble control Control Forecast usually remains within the spread (eg plumes on the right), but on a few occasions (theoretically around 4% at longer lead-times) they extend outside of the plume (eg plumes on the left).  When this happens it is imperative to study the evolution of the atmosphere in some detail to decide the most likely evolution - and definitely not to assume that the unperturbed ensemble control Ensemble Control Forecast results are necessarily the best.  In the case shown, ensemble control Ensemble Control Forecast moved a small cold and potentially snowy vortex northwards across Southampton while the perturbed ensemble solutions moved it past the city either to the east or west.  The subsequent ensemble forecast showed much less uncertainty in temperature and depth of the vortex and it is questionable if the ensemble developed it at all.  Even so, because of the evidence from the earlier forecast, it would be unwise to dismiss the vortex altogether as it cannot be assumed the later ensemble forecast is necessarily the best even though it has benefitted from later observations.

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