The forecast skill for 26 extraordinary Arctic cyclones (EACs) in summer (June-August) during 1986-2016 was assessed using a reforecast data by the National Oceanic and Atmospheric Administration (NOAA)'s second-generation Global Ensemble Forecast System (GEFS). On average, more than 90 of ensemble members predicted the existence of the EACs for lead times of <= 3.0 days. The average central position error for the EACs decreases to <= 433.1 km (half the mean radius of the mature 26 EACs) for a lead time of 3.0 days, with the average central pressure error of similar to 6.9 hPa. The GEFS reforecast for the 26 EACs shows a similar forecast skill to the GEFS reforecast and operational medium-range ensemble forecasts for 10 EACs occurred in summer during 2008-2016. There is no trend of improvements in forecast skills of existence, central pressure and position of the EACs at all lead times. In the verification of the strike probability for EACs within a 400-km radius, the probability information is useful in 1.0- to 5.0-day forecasts, although the forecast probabilities were overconfident for lead times more than 1.0 days.
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