GRIB-2 is highly recommended, but GRIB1 temporarily acceptable.
This site converts GRIB1 to GRIB2 automatically during the upload process.
* GRIB1 ⇔ GRIB2 converison tool
* FYI: NetCDF-GRIB converter is available at
Monthly mean (average from 1st day through the end day of the month)
0-4 months for seasonal forecast
Monthly and/or quarterly