It's actually pretty simple. With the pattern in place, the flow off the Pacific continues to be strong, and is essentially overwhelming other factors. Canada is flooded with relatively mild air. The pattern is a progressive one(no atmospheric blocking in the NA), so the only way to get enough of the already modified cold from up there to down here relies on a well timed, perfectly positioned, but transient NS vortex passing through or near the 50-50 region.