The dataset is typical winter low for 1984-2014. Earth is warming thanks to climate change, and the lowest lows have notably warmed. See, e.g., https://xkcd.com/1321/. In my 12 winters of living in the light blue zone, I think I've spent like a grand total of 36 hours or so in sub-0°F temperatures, and that was pretty much the one period mentioned in the xkcd comic.