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The Global Forecast System (GFS) has been in NWS operations since 1980 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) is at http://www.emc.ncep.noaa.gov/gmb/mission.html. The operational GFS consists of the final Global Data Assimilation System (GDAS), the GFS forecasts, and the Ensemble forecasts (GEFS). 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 T1534 spectral truncation, approximately 13 km physics (Gaussian) grid and 64 levels in a hybrid sigma-pressure vertical coordinate. Output is posted to a quarter degree equally spaced longitude/latitude grid with 3-h forecast interval to 240-h, cycled 4X/day, with 46 vertical standard pressure levels. 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 statistical web page is: http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/ Documentation homepage: "http://www.emc.ncep.noaa.gov/GFS/doc.php