It's a possibility, YYZ has shut down due to snow before but it's rare and unpredictable due to the topography (lake effect snow etc.). I wouldn't sweat it, the chance of it happening to you is remote and there's nothing you can do anyway. In fact thunderstorms are more of a nuisance there in the summer months.

Are you sure there isn't a better award route available ? An East transcon to a transpac is a bit of an epic.