The flood forecast system for the river Sihl in Zurich (Switzerland) is operated by the Federal Research Institute WSL on behalf of the Canton of Zurich since 2007. The close collaboration between forecasters and decision makers fosters continuous development of the system. A management tool was implemented in the system that allows computing scenarios for predicted critical events. The forecast system comprises three forecast chains differing in lead time, spatial resolution and update cycle amongst others. These are the two deterministic chains driven by the numerical weather predictions COSMO-2 and COSMO-7 and the ensemble forecast chain driven by the COSMO-LEPS ensemble. The hydrological forecasts resulting from the three model chains and additional helpful information about the conditions in the catchment are made available on an online platform. The statistical evaluation of the three forecast chains showed that the ensemble forecasts clearly outperform the deterministic forecasts and are more reliable especially for taking decisions that need a lead time of more than just a few hours.