Surprise late afternoon 10m opening to the USA

WSPR reports 10m on 28.12.10

Late this afternoon I was surprised to see that 10m opened up to the USA and that my QRP WSPR beacon was being received well at up to 6991km with reports at 5692km from K3ZV as late as 1800z with my reports of USA stations being the last ones to appear, meaning I was receiving stations when others were no longer doing so. Power was 5W and the antenna the Homebase-10 wire halo.