It was a pretty cold winter (along the lines of -1, 0, -4), but even that isn't very cold in the lower altitudes of the mid-Atlantic. The Dec 19/20th storm's snowfall at DCA, IAD and my backyard was completely washed out by the xmas rain. There was only one day of snow cover in Jan until the storm on the 30th. And then after the big storms, it was basically gone except the piles by the last week of Feb. DCA had 30 days with 1"+ snowcover, and IAD/BWI had 36.
I know this wasn't your main point. It was other posters who brought this idea up about snow cover dwindling away after the big storm(s), BUT, from Ian and Kevin Ambroses' book, here are the resulting snow cover stats from DC's biggest snowstorms:
1) 1/1922- 28"- snow cover lasted 15 days
2) 2/1899- 20.5"- snow cover lasted 11 days
3) 2/1979- 18.7"- snow cover lasted 8 days
4) 2/2010- 17.8"- snow cover lasted 18 days (including the blizzard after of course)
5) 1/1996- 17.1"- snow cover lasted 13 days
6) 2/2003- 16.7"- snow cover lasted 9 days
7) 2/1983- 16.6"- snow cover lasted 8 days
8) 12/2009- 16.4"- snow cover lasted 7 days
9) 2/1936- 14.4"- snow cover lasted 10 days
10) 2/1958- 14.4"- snow cover lasted 8 days
Yes, you have to combine both storms from 2/2010, but the total from both storms (28.6") just barely outdid the Knickerbocker storm and yet the resulting snow cover lasted three more days at the warmer DCA site. I think it's a little harsh to downplay 18 straight days of snowcover at DCA, and much more in the suburbs, even if it resulted from 28.6" of snow in a week.
The other trend from the data makes sense-- after the big event, DC tends to experience a thaw or snowmelt through rain. None of these top 10 events made it to 20 days of snow cover, even ones immediately followed by bitter cold like 2/1958.
A lot of these storms were preceded by other extended stretches of snow cover, but the 30 one-inch snow cover days DCA racked up in 09/10 beat the colder 02/03 (28) as well as 95/96 (25). 78/79 wasn't even close (23), and only '60/'61 (44) beat the total since the move to DCA. Thanks, Ian, for putting together the snow cover days stats.
Other conclusions from looking at the snow cover data:
1) Experiencing only 10-days of winter can still yield a great snow-cover stretch- '99/'00 with 20 days 1"+ at DCA.
2) December and early January moderate snows can be really durable- '89/'90 with 24 days 1"+ and '87/'88 with 22 days 1"+ at DCA.
3) Some of the winters with the most snow cover days did not have a single big (6"+) event during the season- e.g. '62/'63 (27), '76/'77 (24), '77/'78 (25), '89/'90 (24)