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 Fig8.1.7.2 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, and another lying between.  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, HRES/CONTROL (yellow) shows greater 500hPa geopotential height than almost any member of the ensemble.

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 Image RemovedFig8.1.7.3: The ensemble mean and spread for 500hPa heights from the same forecast verifying at 00UTC 28 May21 June.  High standard deviation on right hand chart near 25ºE due to uncertainty regarding the timing and positioning of the major upper trough near 30°E25°E.  The Normalized standard deviation on left hand chart shows greater spread (and hence uncertainty) near 25ºE  than recent ensemble results.

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