Means have been computed for each synoptic hour (00, 06, 12 and 18 UTC) plus a diurnal mean.
The synoptic and diurnal means have been computed for the 6 hour forecast only.  The monthly mean 6 hour forecasts are averaged over all forecasts
<b>valid within the calender month</b>, thus the 00UTC synoptic mean and the diurnal mean each contain a 6 hour forecast initiated at 18UTC on the
last day of the previous month.  The rationale for this choice is that the mean of the 6 hour forecasts coincides with the mean analysis, i.e. it is the mean of the 6 hour background forecasts used in the 3d-Var data assimilation.