Case study of a European medium-range weather forecast bust

Nowadays even medium-range (~6 days) forecasts are mostly reliable but occasionally the quality of the forecasts collapses suddenly. During a collapse or a bust, the actual forecast is worse than a ‘forecast’ made by using climatological mean values. In this study sudden collapse of predictability will be investigated by using one example case from April 2011. OpenIFS NWP model and ERA-Interim reanalysis were used as primary tools. 13 deterministic forecasts with the best available initial conditions were run but one forecast initialized on the worst day is particularly concentrated on. One ensemble forecast of five members also initialized on the worst day is also investigated in this study. Output of OpenIFS was compared to ERA-Interim. Previous studies have shown that the reasons for European forecast busts can be found in North America. Therefore, the aim of this study is to determine if the incorrect representation of convection over North America lead to a forecast bust over Europe. Besides the main goal, this study discusses how the errors originating from North American convection lead to a forecast bust in Europe 6 days later and this study will also be looking for cause of the forecast bust from initial conditions of ensemble forecast. In this case the sudden collapse of predictability in Europe is caused by NWP models predicting change of weather regime wrong. Also OpenIFS predicts formation of a blocking high over Northern Europe although there are no signs of blocking in reanalysis. In Northern America, where the source of the error is, forecast of evolution of a cluster of thunderstorms fails so also convective forcing to large scale dynamics fails. The error grows and is transported to Europe by Rossby waves. Although none of the members of the ensemble forecast was able to forecast the weather properly in Europe, so much deviation was obtained in the outcomes that comparison of the initial conditions was meaningful. The most important finding was that deeper trough over the Rocky Mountains improves the forecast in Europe. This study was able to show evidence that misrepresented convection over North America caused the forecast to fail in Europe. Moreover, this study was able to clarify how the errors caused by misrepresented convection evolved and lead to the forecast bust in Europe. The error at the beginning of the forecast in North America grows so fast that it is unlikely that it would be due to model parameterizations but the initial conditions must contain errors. These failed forecast are difficult to avoid completely but the easiest way to reduce them is to improve quality of the observations in the Rocky mountains.