For us December 2016 was much colder than 2015. Going by HDD's, in 2015 it was about 475 for the month and in 2016 it was about 830.
But of course, even our "colder" monthly HDD's is probably a week's worth for you guys!
By gummeny, you're right! I just checked the past 3 Decembers HDD's for my location, and they are:
December 2014 = 976 HDD's
December 2015 = 705 HDD's
December 2016 = 1,103 HDD's
December 2016 was the coldest in the past 3 years. Mild but colder is how I must sum this up.
It looks like on average I get about 40% more HDD's than you do. But then Rob gets about 25% more HDD's than I get by my reckoning.
Statistical long term average annual HDD's here on my hill at 1,200 ft. elevation are ~6,500, based upon 20 years of records for nearby Chippewa Lake (also at 1,200 ft.). About 25 or 30 miles from here in Akron they only get about 6,350 annual HDD's. So this shows that HDD's can vary quite a bit over regional zones that are not all that far apart.
Statistical trivia: In these parts (at least) the coldest day of the year (on average, long term) comes precisely 1 month after the shortest day of the year, and the warmest day of the year comes precisely 1 month after the longest day of the year.