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M-climate is only re-created each Monday and Thursday.  This can have a small detrimental impact upon theextreme forecast index (EFI) and shift of tails (SOT) values, particularly in spring and autumn when day-to-day climatic changes are at their greatest.

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Fig 5.3.3: The frame on the left shows a sequence Extreme Forecast Index sequence extreme forecast index (EFI) and Shift shift of Tails tails (SOT) charts  charts from ensemble forecasts at 12hr intervals with data times from 00UTC 05 May to 00UTC 09 May all for the same 24hr period 00UTC 09 May to 00UTC 10 May 2017.  Large negative EFI values (Dark Blue, Purple) give strong indication of a significantly cold event compared with corresponding M-climate climatology (shown in the frame on the right).  However, the large negative EFI values (Purple) from forecasts several days ago gradually become less extreme and less extensive (Dark Blues) before returning to large negative EFI values (Purple) on the last forecast.  This is because M-climate has been recalculated reflecting the progressively warmer springtime temperatures experienced in the past.  The forecast temperatures, though not significantly lower than previous forecasts, now appear significantly lower and more extreme compared with the latest M-climate. 

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Fig5.3.4: The frame on the left shows Cumulative Density Function (CDF) of the forecasts in northern Poland with data times from 00UTC 05 May to 00UTC 09 May all for the same 24hr period 00UTC 09 May to 00UTC 10 May 2017 each with an associated colour.  The black line is the M-climate for 24-48hr lead time. The frame on the right shows corresponding extreme forecast index (EFI) values for each ENS run.  On CDF diagrams the M-climate (black) curve jumps to the right (arrow B) during seasonal warming periods and to the left (arrow A) during seasonal cooling periods as the M-climate is reset each Monday and Thursday.  In higher latitudes the most significant changes are in spring and autumn. Changes are much weaker in summer and winter.

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