Predictability of Precipitation and Intraseasonal Variability: Insights From <scp>ECMWF</scp> Model Skill Over Brazil

ABSTRACT This study assesses the capabilities and limitations of the state‐of‐the‐art European Centre for Medium‐Range Weather Forecasts (ECMWF) subseasonal to seasonal (S2S) model in forecasting precipitation and a regional intraseasonal oscillation index over Brazil. Distinct from previous studies, we employ a regional index rather than a global one, enabling a more focused analysis of the complex intraseasonal variability. Weekly accumulated precipitation forecasts are evaluated against satellite‐derived precipitation data for selected regions within Brazil. Our findings indicate that the ECMWF model demonstrates enhanced forecast skill for up to 4 weeks along the northern coast of Northeast Brazil, where tropical–tropical teleconnections linked to the Madden–Julian Oscillation significantly improve predictability. However, forecast accuracy decays after 2 weeks in subtropical and extratropical areas, such as the South Atlantic Convergence Zone and Southern Brazil, primarily due to challenges in capturing synoptic‐scale systems and tropical–extratropical interactions. Additionally, the ECMWF model shows strong predictability for the regional intraseasonal oscillation index up to 10 days, offering valuable insights for planning and decision‐making in the face of extreme weather events. This regional index achieves a level of accuracy not possible with a global index, which is less effective at capturing the intraseasonal signals specific to South America and their impacts on extreme weather.