Pattern-based forecasting enhances the prediction skill of European heatwaves at the sub-seasonal range

Abstract

The prediction of European heatwaves at the subseasonal range is of key importance to mitigate their impact. This study builds on previous work (Rouges et al., 2023) which identifies five main European heatwave types based on their atmospheric circulation patterns (CPs). These CPs are potential predictors of heatwaves, as these patterns are connected with a high probability of 2-meter temperature exceeding the 90th percentile. Therefore, the aim of this study is to use these patterns to construct a pattern-based forecast method. The skill of this method to forecast extreme warm temperatures is then assessed and compared with the direct grid-point based forecast (using the direct 2-meter temperature forecast of the model). The extended (or subseasonal) range reforecast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) is used for the skill evaluation. Firstly, the skill of the extended range model is assessed in predicting CPs. This methodology is then compared with the direct prediction of extreme warm temperatures. The results show that the pattern-based methodology has a low skill at the short to medium range compared to the direct method, however it maintains skill for longer lead times, extending the forecast skill horizon significantly by up to six days over key heatwave regions. This improvement is localized over regions with the highest conditional probability of extreme warm temperatures. Further on, the prediction skill of persistent high temperatures (four days) is also assessed using persistent CPs (five days or longer). Similarly, an improvement in forecast skill horizon is observed but the improvement is more modest and even more localized. The extension of the forecast skill horizon seen at the subseasonal range with the pattern-based forecast method constructed in this study could be used to provide early warnings of European heatwaves and therefore support the timely implementation of mitigation plans.