What mkasner said!
I try to get to it as soon as possible. If not, snow will melt right atop the roof and freeze. If it get bad enough it will actually start (a la ice dams on a house roof eave) to collect and lift roof components if they have any lip of sorts. Vents, AC cover, Heki skylight on my TC are susceptible to it.
If I can get the snow off then generally the next sunny day will allow the ice to melt from direct sunlight even if the temps are below freezing. 2 weeks ago was not too bad even though we had 18+ inches... the wind was so intense that I had no snow to sweep.
Not to hijack the topic, but has anyone who lives where dry powder snow is common- have you ever used a leaf blower to clear the roof?