A Combined Method for Forecasting Extreme Monthly Precipitation in Europe

In a first step, we study the possibility to use the 1-month Standardized Precipitation Index (SPI-1) for forecasting the probability of extreme monthly precipitation events over the next 30 days. To do so, the performances of the first month of the ECMWF probabilistic extended forecast to predict extreme events of precipitation over Europe is analysed. Results demonstrate that around 40% of droughts in Europe are correctly forecasted one month in advance, but the extreme positive anomalies are more difficult to predict. Moreover, false alarms or misses are still important.

In order to improve the significance of the precipitation forecast, in a second step we analysed the possibility of adding atmospherical predictors, such as the occurrences of specific weather regimes in winter and of blocking situations in summer. Firstly, the occurrences of different weather types using ERA Interim reanalysis and extreme observed SPI-1 are studied to highlight potential relationships. Then , by using Discriminant Analysis, the occurrences of weather regimes in winter or blocking situations in summer are used to predict extreme events of monthly precipitation in Europe. Over several parts of Europe (especially in the Northern part), results show strong links. Due to their spatial structures, extreme dry periods are still better forecasted than the extreme wet periods. Finally a combined method, using the SPI forecast and the predictors, is proposed to produce an early warning of extreme events in Europe.