Forecast data provided by European (ECMWF, DWD etc) , U.S. organizations (NOAA/NWS) and/or National Meteorological Services, usually in the form of GRIB files.
For more info:
https://community.windy.com/topic/3340/when-and-how-often-are-the-weather-data-updated
For GRIB files:
https://en.wikipedia.org/wiki/GRIB