Hi NS
FYI we don't have to carry extra fuel (but yes, it would be a good idea to! - but take a long haul flight, they might not be able to get the extra fuel on board) as long as we meet our planning requirments at dispatch. As far as our rules are concerned it is the depth of contaminent that is relavant, the BA is ONLY used for the crosswind limit.
The reason I ask is that this very same senario happen a couple of yrs back in the London TMA - many pans called becuase of the snow - the aircraft (at least ours) we able to land with the current depth of snow - thats my point. Taking scandinavia as an e.g. the weather may be such that you CAN'T find a decent alternate for a considerable distance and therfore, you might not be able to load up the extra fuel.
v. intersting though.