One of the largest known irruptions of Northern Hawk Owls Surnia ulula in southern Norway started in mid August 2016. During September-October I conducted 246 km surveys in boreal forests in Oslo and Akershus counties, southeastern Norway. A total of 16 owls were detected. Depending on assumptions regarding detectability, a density of 0.106-0.204 owls/km2 was estimated, corresponding to 371-714 individuals in boreal forests of the two counties in total. Southeastern Norway has about 55,600 km2 boreal forest which implies a total of 5,894-11,342 owls in this habitat. On the bird reporting websites artsobservasjoner.no and nofoa.no there were 1,366 reports of Northern Hawk Owls from all counties in southern Norway during September-October, representing 661 different sites. In Oslo and Akershus counties, estimates suggested that < 10% of all hawk owls were detected. A 5-10% detection rate suggests that there were 6,610-13,220 owls in southern Norway. However, in many counties hawk owl reporting rates measured as proportion of all bird reports that concerned hawk owl, were much higher than in Oslo and Akershus, which could imply higher density. By adjusting density (using the values 0.106-0.204 owls/km2 from boreal forest in Oslo and Akershus) for county-specific owl reporting rates, the number of hawk owls present only in boreal forest in southeastern Norway was estimated at 14,466-27,872 individuals. This number excludes additional owls in agricultural areas and owls in other parts of southern Norway. Overall, a total number of about 10,000-20,000 individuals is considered most likely. I review available evidence of the origin of these owls using number of reports and spatial distribution of hawk owls in northern Norway, Sweden, Finland, Estonia and Russia during 2015 and 2016. I conclude that the hawk owls came from northern Fennoscandia where large numbers of Northern Hawk Owls bred in 2015, and not from more eastern areas such as Russia.