Those are just records for a particular day in December.
[link|http://www.intellicast.com/Almanac/MidAtlantic/December/|http://www.intellica...tlantic/December/] - Record December low in DC is -13 F (it doesn't say when though).
I've seen the Potomac frozen over, but not in the last couple of years. It's been pretty mild up recently, until this year. Brr.
HTH.
Cheers,
Scott.