Problem
If you have a file with both Control and Perturbed forecast in the same GRIB file, grib_to_netcdf will only convert the Perturbed Forecast. From 51 members present in the GRIB file, only 50 appear in the netCDF file.
$ grib_to_netcdf -o output.nc -s input.grib
...
grib_to_netcdf: Found 51 GRIB fields in 1 file.
...
grib_to_netcdf: Done.
$ ncinfo output.nc
...
dimensions(sizes): longitude(26), latitude(31), number(50), time(1)
This only occurs when 'type=pf and type=cf' fields coexist on the same file (common use case if you get real-time data).
Solution
The solution is to set the type=pf for the control forecast. Once converted the Control Forecast will appear with 'ensemble_number' = 0 in the NetCDF file.
grib_set -s type=pf -w type=cf input.grib input2.grib grib_to_netcdf -o output.nc -s input2.grib