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 are the Standardized Precipitation Index (SPI) and the threshold level method (TLM).
The SPI provides the drought index 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 (SPI-x, 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 severity (deficit volume), duration, onset, and termination of the drought in precipitation in probabilistic terms. The drought occurs when the precipitation is below the threshold. Summaries of total drought area (SPI and TLM), total drought duration (SPI and TLM), and total drought deficit volume (TLM) are provided.
The probabilistic forecasts are presented as 25th percentile, median, and 75th percentile of the 51 ensemble members.