Global Forecast System
The mathematical model is run four times a day, and produces forecasts for up to 16 days in advance, but with decreased spatial resolution after 8 days. The forecast skill generally decreases with time (as with any numerical weather prediction model) and for longer term forecasts, only the larger scales retain significant accuracy. It is one of the predominant synoptic scale medium-range models in general use.
The GFS model is a spectral model with an approximate horizontal resolution of 27km for the first 8 days and 35km from 192 to 384 hours (16 days). In the vertical, the model is divided into 64 layers and temporally, it produces forecast output every hour for the first 24 hours, every 3rd hour out to 8 days, after that they are produced for every 12th hour. In 2014 an upgrade to the system is planned to increase the resolution to about 13km out to 10 days. The output from the GFS is also used to produce model output statistics, in three ranges: every hour for 24 hours, every three hours out to three days, and every 12 hours out to eight days.
In addition to the main model, the GFS is also the basis of a lower resolution 20-member (22, counting the control and operational members) ensemble that runs concurrent with the operational GFS and is available on the same time scales. This ensemble is referred to as a "Global Ensemble Forecast System" (GEFS). Ensemble model output statistics are also available out to 16 days. The GFS ensemble is combined with Canada's Global Environmental Multiscale Model ensemble to form the North American Ensemble Forecast System (NAEFS).
As with most works of the U.S. government, GFS data is not copyrighted and is available for free in the public domain under provisions of U.S. law. Because of this, the model serves as the basis for the forecasts of numerous private, commercial and foreign weather companies.