A regional North American plot of GEFS forecast data valid at July 13, 2012, at 00 UTC. This 24-hour forecast shows one mean sea-level pressure contour (or isobar) at 1013 hPa for each of the 21 members in the ensemble. Where contours line up show greater certainty in the pressure forecast. Where the contours spread out show uncertainty: over central, northern Canada and due south of Alaska, for example. This image was produced by downloading 21 files of GEFS data through NOMADS and visualizing with the Grid Analysis and Display System (GrADS).
The Global Ensemble Forecast System (GEFS), previously known as the GFS Global ENSemble (GENS), is a weather forecast model made up of 21 separate forecasts, or ensemble members. The National Centers for Environmental Prediction (NCEP) started the GEFS to address the nature of uncertainty in weather observations, which is used to initialize weather forecast models. The proverbial butterfly flapping her wings can have a cascading effect leading to wind gusts thousands of miles away. This extreme example illustrates that tiny, unnoticeable differences between reality and what is actually measured can, over time, lead to noticeable differences between what a weather model forecast predicts and reality itself. The GEFS attempts to quantify the amount of uncertainty in a forecast by generating an ensemble of multiple forecasts, each minutely different, or perturbed, from the original observations.
With global coverage, GEFS is produced four times a day with weather forecasts going out to 16 days. Gridded data are available through the NOAA National Operational Model Archive and Distribution System (NOMADS). NOMADS also contributes GEFS ensemble data to the The Observing system Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) by calculating a dozen World Meteorological Organization (WMO)-required variables and passing them to the National Center for Atmospheric Research (NCAR) for permanent archive. NOMADS also provides an additional tool, the NOMADS Ensemble Probability Tool, which allows a user to query the multiple forecast ensemble to determine the probability that a set of conditions will occur at a given location using all of the GEFS ensemble members in near–real time. Many other forecast products are available at the GEFS home page.
Offline data ordering of GEFS with the HDSS Access System (HAS) requires checking the "Bulk Request" option. While this is not enforced by the site, requests submitted without the bulk option are not supported and will not return data. Bulk dataset requests are large and cannot be subsetted, so start with a small request (one day of data) to ensure you find it usable.