The Global Forecast System (GFS) spectral model was originally developed by J. Sela (Sela 1982, 1988), and is continuously improved by the Global Climate and Weather Modeling Branch which conducts a program of research and development in support of the Environmental Modeling Center (EMC) (http://www.emc.ncep.noaa.gov/) of the National Centers for Environmental Prediction (NCEP http://www.ncep.noaa.gov/). The operational forecasting mission for global prediction for medium range (3-14 days) and for extended range (week2 – S/I) is at http://www.emc.ncep.noaa.gov/gmb/mission.html. The operational GFS consists of the final Global Data Assimilation System, the GFS forecasts (GFS), and the Ensemble forecasts (ENS). The GFS is a consolidation of the forecasts formerly known as the Aviation (AVN) and the Medium Range Forecast (MRF). The model is based on the usual expressions of conservation of mass, momentum, energy and moisture at T382 spectral truncation, approximately 0.3 degree physics (Gaussian) grid and 64 levels in a hybrid sigma-pressure vertical coordinate. Output is posted to a 1x1 degree equally spaced longitude/latitude grid with 3-h forecast interval to 180-h, cycled 4X/day, with 26 vertical standard pressure levels called the "GFS high resolution" data set in NOMADS. See History of recent modifications to the global forecast/analysis system at: http://www.emc.ncep.noaa.gov/gmb/STATS/html/model_changes.html The model performance statstiscal web page is: http://www.emc.ncep.noaa.gov/gmb/STATS/STATS.html More technical information can be found in the Parallel runs home page: http://www.emc.ncep.noaa.gov/gmb/para/parahome.html Documentation homepage: http://wwwt.emc.ncep.noaa.gov/gmb/moorthi/gam.html Source code at: http://www.nco.ncep.noaa.gov/pmb/codes/nwprod/sorc/