I have found the same elements than you and also went for a nuclear winter. I had a very severe one after the exchange with temperatures dropping by 20°C in several places.
Then, I maintained this situation overtime with more severe winter and shorter summer. An other thing is that extreme weather conditions are reinforced in several locations (you don't get rain in deserts). As a result, in US some places are colder but the deserts are experiencing more severe drought than usual. I'm not really sure that my choices are entirely realistic but they are fun in game terms and I love to have my players walking in snow from early November to late April.
