Drought forecasting is based on forecasted gridded time series of precipitation and evaporation (climate water balance) for the whole of Europe and at regional scale. The gridded time series are obtained from EFAS (driven by IFS-SEAS). The Standardized Precipitation Evaporation Index (SPEI) is used as approach.
The SPEI provides the severity of the drought in the climate water balance (difference between precipitation and evaporation) as the number of standard deviations from the median (e.g. SPEI=-2 says that the difference between the precipitation and evaporation is 2 standard deviation from the median). The SPEI aggregates the precipitation-evaporation over periods with different lengths (SPEIx, x = 1, 3, 6, and 12 months). For instance, the seasonal forecast of SPEI-12 gives the probability of the total difference between precipitation and evaporation over 12 months, that is, 7 months ahead and the 5 previous months.
The SPEI also provides the total drought affected area. The probabilistic forecasts are presented as 25th percentile, median, and 75th percentile of the 51 ensemble members.