Meteorological applications of extreme value statistics are often limited by the relative shortness of the available datasets. In order to overcome this problem, we use archived data from all past seasonal forecast ensemble runs of the European Centre for Medium-Range Weather Forecasts (ECMWF) for estimating low frequency return values. For regions where the forecasts have very little seasonal skill the archived seasonal forecast ensembles provide independent sets that cumulate to over 1500 years. We illustrate this approach by estimating 104-year sea-surge levels at high-tide along the Dutch coast. In comparison with the observational sets, the ECMWF set shows a decrease in the statistical uncertainty of the estimated 104-year return value by a factor four