Lauriane Batté

This study proposes an objective methodology to highlight windows of opportunity in a numerical subseasonal forecasting system. The methodology is based on a contingency table and is applied to the prediction of heavy tropical precipitation by the European Centre for Medium‐range Weather Forecasts (ECMWF) subseasonal‐to‐seasonal (S2S) reforecasts in the November‐to‐April season, in relation with the Madden–Julian oscillation (MJO). As a slowly propagating signal of enhanced convection, the MJO may indicate favorable conditions for heavy precipitation a few weeks ahead in some tropical areas. The combined knowledge of these climatological impacts and the current phase of the MJO at initialization defines observation‐based “climatological windows of opportunity.” We then investigate whether the ECMWF S2S forecasts are indeed more performant when there is increased likelihood of heavy rainfall, that is, whether the model converts “climatological windows of opportunity” into “model windows of opportunity.” Our results show that, by Week 2, this is only verified for a limited number of tropical areas, mostly located in the western Pacific and Africa. Meanwhile, failures to seize the opportunities lie in misplaced MJO impacts, signal loss, or too many false alarms.