Atmospheric blockings are large-scale, persistent flow patterns which may be linked high impact weather. Their predictability is low, especially during their onset and decay phase. In summer 2010, an enormous blocking anticyclone evolved over Russia and caused a heat wave in Russia and Eastern Europe as well as floods in Pakistan. In this study, the forecast variability of the 2010 summer block is investigated in medium-range as well as subseasonal ensemble forecasts. In addition, dynamical processes which are responsible for the forecast variability of the block and resulting surface impacts are analyzed.