For those who are interested, some good background from the ECMWF. Also notable is the fact that winter forecasts tend to be more reliable due to longer wavelengths.
https://www.ecmwf.int/en/newsletter/153/meteorology/25-years-ensemble-forecasting-ecmwf
Evolution of ensemble forecast quality
Thanks to model upgrades, improvements in the data assimilation system, the use of more observations, and the ENS configuration changes discussed above, the ENS performance has increased substantially during the past 25 years.
Figure 7 Time evolution, from 1 January 1995 to today, of the Continuous Ranked Probability Skill Score (CRPSS) of ENS forecasts of 500 hPa geopotential height over the northern hemisphere, for lead times of 24 hours, 72 hours, 120 hours, 168 hours and 240 hours. Forecasts are verified against operational analyses. The more or less regular pattern of peaks and troughs in each line stems from differences in predictability related to the seasons: winter weather tends to be more predictable than summer weather.