452: Forecast Ensembles

Tables of an ensemble of forecasts for individual gridpoints/stations/series and a graph of the historical and current forecast(s) are shown. The table shows an ensemble of forecast values for the current variable and forecast target period.

The ensemble is generated by defining equi-probable intervals between the forecasts, based on the cross-validated (by default) error distribution. The default number of ensemble members is nine, so that there is a 10% probability of the observed value falling between and two sorted member, and a 10% probability of the observed value being larger than the largest member, and another 10% probability of it being smaller than the smallest member. The number of ensemble members can be reset using Options ~ Forecast Settings ), as can the method for calculating the error variance. The retroactive and fitted error variances are alternative options.

The graph shows the history of the cross-validated forecasts (green line) and observations (red line), and the current ensemble of forecast(s) shown by green crosses. The prediction intervals for the forecasts can be added by right-clicking on the graph, and following the "Customise" option. The vertical coloured divisions show the three categories. Note that when calculating the cross-validated forecasts, the threshold definitions will have fluctuated slightly, and so the current categorisation of the forecasts and observations may not exactly match the cross-validated classification. This fluctuation is the reason for any apparent discrepancies between the graph and the contingency tables .

The forecasts can be saved by selecting the Forecasts tab from the File _Output Results ) menu. The graph itself can be exported as a graphics file by right-clicking anywhere in the child window and selecting Export from the pop-up menu. A default name is given to the graphics file, but this name can be changed using the browse button. The image quality for all the graphics formats can be improved by increasing the size of the image (see Options ~ Graphics ~ Graph Scaling ). The graphic title can be set using the Customise option upon right-clicking in the child window.

The y-axis limits can be set by right-clicking on the image and then selecting the Customise ~ Y-Axis Limits option. The maximum and minimum values on the y-axis are indicated and can be reset. Resetting these axis limits will in turn automatically reset the tick-mark interval and number of tick marks. Greater flexibility for setting the tick marks is planned for future releases. Further options for customising the graph, including adjusting the size of the fonts, and the size of the images, are available from Options ~ Graphics .