To sum up your New Year's Eve forecast, "colder air will be pouring into the East Coast, right along with the champagne!"
Not that it will "bitterly cold" by any stretch of the imagination. But, with lows on New Year's morning reaching down to near freezing, and a daytime high temp of just 45F or so on New Year's Day, you will definitely feel the change!
Additionally, you can expect the fresh, seasonably January chill to hold up right through the long weekend and into early next week.
Of course, this doesn't mean we won't see any more unseasonably mild spikes after that, deeper into January. But overall, it does look like we may finally be seeing a shift to a colder pattern, and just in time for the start of our statistically coldest month!
Have a great New Year's everyone (and stay warm)!