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 drought index in the climate water balance (difference between precipitation and evapotranspiration) as the number of standard deviations from the median (e.g. SPEI=-2 says that the difference between the precipitation and evapotranspiration is 2 standard deviation from the median). The SPEI aggregates the precipitation- evapotranspiration 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 evapotranspiration over 12 months, that is, 7 months ahead and the 5 previous months. Summaries of total drought area and total drought duration are provided.
The probabilistic forecasts are presented as 25th percentile, median, and 75th percentile of the 51 ensemble members.