The Air Traffic Flow Management Daily Plan is published by Air Services the night before. If adverse weather is predicted, then the rate of arriving and departing aircraft is reduced. This then causes cancellations as there are more flights scheduled than available runway capacity.
This is why you'll see cancellations start to trickle in hours in advance, as the airlines pre-emptively cancel a number flights to manage the overall operation based around the reduced arrival/departures rates or ground delay programs that are going to be in place.
The other option is they wait until the last minute to cancel flights, knowing full well that runway capacity is reduced and cancellations are inevitable. I know I'd prefer the advance notice...
