Nuclear winter is a hoax. Bombs below a certain yield cannot rise to the required altitude. And bombs above that yield don't linger at the target long enough to gather soot from burning wreckage. We have had worse from volcanoes.

My suspicion is a faked radioactivity release, from a purported detonation over the Atlantic Ocean, blowing in to the Northeast. No way for the common person to confirm it (get your own surplus Geiger counter; confirm your background level). Just as good as---maybe even better than---a fake disease, from the standpoint of imposed control and probably martial law. It would affect a LOT of people (millions and millions). That would be the center of "damage control"...which would really be disinformation control. "Trust us," and the rest of the nation would just have to sit there, twiddling thumbs. Best if timed to displace the fall election. Think "contaminated" mail-in ballots? Suspension of in-person balloting?

Might be a good idea to popularize this scenario projection in order to "spoil" the movie.