Drought forecasting is based on forecasted gridded precipitation time series (EFAS, IFS-SEAS) for the whole of Europe and at regional scale. Two approaches are used, that is the Standardized Precipitation Index (SPI) and the threshold level method (TLM).
The SPI provides the severity of the drought in precipitation as the number of standard deviations from the median (e.g. SPI=-2 says that the precipitation is 2 standard deviation from the median). The SPI aggregates the precipitation over periods with different lengths (SPIx, x = 1, 3, 6, and 12 months). For instance, the seasonal forecast of SPI-12 gives the probability of the total precipitation over 12 months, that is, 7 months ahead and the 5 previous months.
The TLM forecasts the onset, duration, and severity (deficit volume) of the drought in precipitation in probabilistic terms. The drought occurs when the precipitation is below the threshold.
The SPI also provides the total drought affected area. The probabilistic forecasts are presented as 25th percentile, median, and 75th percentile of the 51 ensemble members.