GRIB-2 is highly recommended, but GRIB1 temporarily acceptable.
This site converts GRIB1 to GRIB2 automatically during the upload process.
* GRIB1 ⇔ GRIB2 converison tool
http://www.nco.ncep.noaa.gov/pmb/codes/GRIB2/
http://www.ecmwf.int/products/data/software/grib_api.html
* FYI: NetCDF-GRIB converter is available at
http://www.mpimet.mpg.de/fileadmin/software/cdo
Monthly mean (average from 1st day through the end day of the month)
0-4 months for seasonal forecast
Monthly and/or quarterly