Three layers go into the answer above. For the next two hours we use a 15-minute precipitation nowcast, which works much like radar extrapolation — that is what lets us say rain starts in twenty minutes rather than "this afternoon". Beyond that comes the hourly forecast — Environment Canada's for the first 24 hours, model data after that — and we flag any hour with a 40% chance or more, or with measurable rain. For the rest of the week the daily probabilities take over. A 40% chance does not mean it rains for 40% of the day; it means there are four chances in ten that measurable precipitation falls at a given spot in the area during that period. The most useful trick is still the oldest one: watch the radar. If a band of rain is moving toward you at 50 km/h and sits 100 km away, you have about two hours, whatever any percentage says.