Global Forecast System
The Global Forecast System (GFS) is a global numerical weather prediction computer model run by NOAA. This mathematical model is run four times a day and produces forecasts up to 16 days in advance, but with decreasing spatial and temporal resolution over time. It is widely accepted that beyond 7 days the forecast is very general and not very accurate, and most nongovernmental agencies rarely use any of the model's results beyond 10 days (mainly because there is no other 16-day model with which to compare). Along with the ECMWF's Integrated Forecast System (IFS) and the Canadian Global Environmental Multiscale Model (GEM), both of which run out 10 days, it is one of the three predominant synoptic scale medium-range models in general use.
The model is run in two parts: the first part has a higher resolution and goes out to 180 hours (7 days) in the future, the second part runs from 180 to 384 hours (16 days) at a lower resolution. The resolution of the model varies in each part of the model: horizontally, it divides the surface of the earth into 35 or 70 kilometre grid squares; vertically, it divides the atmosphere into 64 layers and temporally, it produces a forecast for every 3rd hour for the first 180 hours, after that they are produced for every 12th hour. The GFS is also used to produce model output statistics, both in a short range (every 3 hours, out to 72 hours) and in an extended range (every 12 hours, out to 8 days).
In addition to the main model, the GFS is also the basis of a 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 is variously referred to as a "Global Ensemble Forecast System" (GEFS or GENS) or the "Medium Range Forecast" (MRF). Ensemble model output statistics are also available out to 8 days.
This is the only global model for which all output is available, for free in the public domain, over the internet (as a result of U.S. law), and as such is the basis for non-state weather companies, e.g., Weather Underground, AccuWeather, The Weather Channel and MeteoGroup. (The IFS only has a limited amount of its output available for free, and all of the GEM's content remains under copyright.)