Sustained has always seemed uncommon. I mean most winters either see the cold or mild dominate, but its rare for anything to truly be sustained. I definitely agree that snow is not a way to determine if a winter is cold or mild...and thats why I brought up the rarity of a winter that is both simultaneously abnormally cold AND snowy. For instance, the winter of 1977-78 from a snowfall standpoint essentially died after the Jan 26 blizzard, which is only mid winter...but I don't think anyone would not call that a "cold, snowy winter".
That does give me an idea though...I do want to look up later how many winters saw all 3 months finish warmer or colder than avg.
btw if anyone is interested, here is a list of nina winters since 1950