A regional North American plot of GEFS forecast data valid at July 13, 2012 at 00UTC. This 24-hour forecast shows one mean sea-level pressure contour (or isobar) at 1013hPa 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) 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 are 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 NOMADS. NOMADS also contributes GEFS ensemble data to the THORPEX Interactive Grand Global Ensemble (TIGGE) by calculating a dozen WMO-required variables and passing 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 homepage.
A screen capture of output from the NOMADS Ensemble Probability Tool. This graphic shows the probability that cloud cover will be greater than 50% (that is, either mostly cloudy or overcast) at Reagan International Airport in Washington, DC from July 12, 2012 at 06UTC through July 28, 2012 at 00UTC—a 16-day forecast. The graphic shows it likely to have mostly cloudy to overcast skies for the first four and a half days of the forecast period, then becoming more likely to have partly cloudy to clear skies for the remainder of the forecast period.