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