The performances of the first month of the ECMWF probabilistic extended forecast and the seasonal forecast to predict droughts over Europe are compared. The Standardized Precipitation Index is used to quantify droughts. It can be shown that on average the extended forecast has higher skill than the seasonal forecast whilst both outperform climatology. No significant spatial or temporal patterns can be observed but the scores are improved when we focus on large scale droughts.
This report further analyses several different methods to convert the probabilistic forecasts of SPI into a Boolean drought warning. It can be demonstrated that methodologies which convert low percentiles of the forecasted cumulative distribution function of SPI into warnings are superior in comparison to alternatives such as the mean or the median of the ensemble. This work demonstrates that around 40% of droughts in Europe are correctly forecasted one month in advance. Nevertheless, due to the lack of the significant difference between the distributions of the ensemble members for false alarms or misses on one hand side and correct forecasts on the other hand side, it is not yet possible to quantify the uncertainty of the drought forecasts.JRC.H.7-Climate Risk Managemen
