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Like the NAM-NMM, the Global Spectral model is run four times a day. Each model run is for 16 days, much longer than the other models.
As the name implies, this model is a spectral model rather than a grid point model in the horizontal, and also covers the entire globe as opposed to the NAM-NMM with a domain that only covers North America. The model has an approximate horizontal resolution of 13 km for the first 10 days, and 27 km from 240-384 hours. Like the NAM-NMM, it uses a hybrid sigma-pressure vertical coordinate system; however the GFS makes use of the hydrostatic approximation. The GFS has 64 vertical levels.
While it is the only model available for longer-range forecasts, it often produces the best forecast at shorter range as well. Because of its global domain, the GFS is also used to determine the boundary conditions for the NAM-NMM.
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