Model: GFS Ensemble from the American Weatherservice Update: 4 times per day, from 00:00, 06:00, 12:00 und 18:00 GMT Greenwich Mean Time: 12:00 GMT = 13:00 BST Variable: Geopotential Height at 500 hPa Description: An ensemble forecast is a set of different forecasts produced by numerical weather prediciton (NWP) models all valid at the same forecast time(s). The forecasts in the ensemble may have different initial conditions, boundary conditions, parameter settings, or may even be from entirely independent NWP models. Plotted together in one diagram or map the differences between the forecasts (aka: model runs) can provide information on the probability distribution of the predicted variables, resp. the weather. The wide red lines represent the ensemble average of all model runs. This ensemble average is usually more accurate than any single model run. The distance between the ensemble average and each individual line (model run) is known as the ensemble spread which is used to estimate the confidence of a specific weather forecast. Confidence is highest when the lines lie closely packed together.