Forecasting Severe Weather

An illustration of ensemble forecasting - The National Centers for Environmental Prediction routinely present the results of ensembles of forecasts in order to convey to users the uncertainty in forecasts for various ranges. For example, an ensemble of forecasts of 500 mb height is displayed in a so-called "spaghetti plot" (so-called because of its resemblence to spaghetti). Two different 500 mb height contours (one in red, one in blue) from all members of an ensemble of (presently) 23 forecasts are plotted in the diagrams. This animation is for a 15-day period from an ensemble of forecasts made by the National Weather Service's GFS model. On the first day of the forecast, the contours from the different runs are close together in most locations, essentially overlapping each other. However, as the forecast extends beyond a few days, the contours spread out, giving the plot its "spaghetti" appearance. The rate of spreading varies geographically, implying that the uncertainty increases at different rates in different regions. For example, at days 7 and 8 (168 and 192 hours), the forecasts for the eastern North Pacific and North America are much closer together than over Europe and the Mediterranean. Even in the initial analysis (the "Day 0" forecast), you will notice a slight separation of the 500 mb contours; this separation represents the initial differences that are prescribed in order to create the differences in the forecasts comprising the ensemble.