@sleeper, there is always the odd exception. The 737NG is approved to take off with some limited contamination on the upper wing surface. However not the kind you get from any kind of snowfall which is usually spread more or less evenly over the whole wing. It is just a provision to allow some frost accretion with a cold soaked wing in humid weather.
@SGC, in that case you still have a hold-over time which is pretty limited with the high temperature and continuous snowfall.