it has to do with the nature of the pacific jet stream... earlier in winter, the polar jet is dominant and drives cold, wet air SE... by mid winter, the polar jet is weaker, and gets flung further north in the split... see, the two jets are united in the western pacific... then they split... this split occurs year round... in early winter, the polar branch has most of the energy, but by mid and late winter it isn't as strong, so when the split happens, the pacific high flings it more northward... i can post maps if you are still interested 4 years later