Between May and July 2015, substantial seasonal variability was observed over Sweden compared to the standard conditions expected for this region and period. As a result of below-normal temperatures and solar radiation, and increased precipitation delayed snowmelt occurred in the Umeälven river basin.

Underestimation of snow availability, in combination with high precipitation in late July, resulted in the inaccurate prediction of the water inflow to the reservoirs and unproductive water release, leading to significant economic loss for hydropower generators.

In the S2S4E project, seasonal forecasts for precipitation, inflows and snow water equivalent were produced for 1, 2 and 3 months before the spring flood period (May-July). The hydrological forecasts (snow water equivalent and river inflows) correctly predicted values that are not observed under normal conditions for this period and region at all lead times. For precipitation, forecasts with a lead time of 1 and 2 months showed a high probability of exceeding the usual conditions, but predictions with high lead times (3 months) were shown to be unreliable.

Therefore, since snow can strongly impact river inflows, seasonal hydrological forecasts are considered robust for decision-making in the hydropower sector to manage the water reservoirs throughout the year effectively.