RTOFS v3.0 proposal: UFS/MOM6/CICE6 and GRIB2 filename changes

NOAA/NWS issued PNS 26-17 on March 12, 2026, soliciting comments through April 15, 2026 on a proposed Real-Time Ocean Forecast System (RTOFS) v3.0 upgrade.

The proposal replaces the current HYCOM ocean model and CICE4 sea-ice model with UFS/MOM6 and UFS/CICE6. NOAA says RTOFS would continue to publish GRIB2 and NetCDF products through NOMADS/FTPPRD.

What changes

RTOFS v3.0 moves the operational ocean/ice system toward the same UFS component stack used elsewhere in NOAA modeling:

• HYCOM is replaced by MOM6
• CICE4 is replaced by CICE6
• legacy binary and compressed products tied to HYCOM/CICE4 are removed
• output continues in GRIB2 and NetCDF

NOAA says the following product families would be discontinued:

• binary-formatted meteorological fields (*.a, *.b, *.B) used by the old CICE4 component
• HYCOM native archive files (*.a, *.b, *.B)
• tar-compressed HYCOM/CICE4 binary files (*.tgz)
• compressed sea-surface temperature and velocity files (*.tar.gz, *.gz)

The notice says the old binary meteorological fields contained GDAS/GFS fields ingested by CICE4. In RTOFS v3.0, CICE6 reads NetCDF-formatted GDAS/GFS fields directly.

GRIB2 filenames change

The GRIB2 products stay, but the filename pattern changes.

Current forecast files:

rtofs_glo.t00z.f{HHH}_{region}_std.grb2

New forecast files:

rtofs_glo.t00z.f{HHH}.{region}.std.grib2

Current nowcast files:

rtofs_glo.t00z.n024_{region}_std.grb2

New nowcast files:

rtofs_glo.t00z.tm000.{region}.std.grib2

For users who scrape NOMADS paths or build indexes from file names, that rename is the main operational detail.

What this means for GribStream users

GribStream does not currently expose RTOFS as a model. This proposal still matters because RTOFS keeps GRIB2 products on NOMADS, and GribStream intends to mirror and serve GRIB data that is available on NOMADS but not in public S3 buckets.

It also touches model families GribStream already serves: RTOFS v3.0 still uses GDAS/GFS fields as upstream forcing/input data, but the notice does not say that GFS or GDAS outputs themselves are changing. The impact is on RTOFS product structure and filenames.

If NOAA proceeds, the final Service Change Notice should give at least 30 days notice before implementation. The main things to watch are the final date, NOMADS path layout, and whether the .grib2 rename lands exactly as proposed.

Sources

• NWS PNS 26-17, "Upgrading Real-Time Ocean Forecast System (RTOFS) to version 3.0" (Mar 12, 2026): https://www.weather.gov/media/notification/pdf_2026/pns26-17_RTOFS_v3.pdf
• NWS notification index: https://www.weather.gov/notification/