How are the averages of surface temperature in the NOAA NCEP-NCAR CDAS-1 MONTHLY Diagnostic dataset computed? Is it the monthly average of daily snapshots at noon or of every 6-hour time step?

You may want to take a look at the official NCEP/NCAR Reanalysis documentation. What I read in the data summary seems to imply that the monthly mean diagnostic surface temperature values are calculated from the " ... instantaneous values at the end of the 6-hr forecast."

-- Michael Bell