Since windyty appears to be using GFS, NEMS, and NAM model data, my guess is that they are using the GRIB files from those respective sources and aren't building new GRIB files. The NOMADS server at NOAA has the GFS and NAM GRIB files. The NEMS data may be from the meteoblue API.
Posts made by inomyabcs
RE: GRIB Files
RE: What source of weather data Windy use?
@DaveySoCal Yes, the weather models use observed sources that are available at the time of ingest. Common ingest sources are RADAR, satellite, aircraft reports, upper air soundings (weather balloons), ground stations, and ocean buoys. You can read more about the ingest data sources for the GFS here (it is the same for the other models), http://www.emc.ncep.noaa.gov/GFS/doc.php paragraph 1.3 (GDAS).