Combinations of large-scale circulation anomalies conducive to precipitation extremes in the Czech Republic

Heavy regional precipitation events are frequently associated with certain circulation conditions in central Europe. To account for the variability of the synoptic conditions leading to heavy precipitation, reference events were divided into four synoptic variants. We demonstrate that the causal circulation conditions can be characterized by combinations of anomalies of properly selected meteorological variables. Using the ERA-40 database, anomalies in the meteorological fields that have repeatedly occurred during historic heavy precipitation events in the Czech Republic were detected. Afterwards, we linearly combined individual predictors and created the Circulation Extremity Index (CEI). Independent re-analyzed data confirmed that high values of the CEI were always associated with heavy precipitation events; however, such events were not always accompanied by high values of the CEI. If calculated from Numerical Weather Prediction outputs, then the CEI would be independent of uncertain Quantitative Precipitation Forecasting and could support risk management decisions.