Forecast models indicate while Sunday should begin sunny and actually fairly warm, a line of showers and thunderstorms could move through the Seattle area Sunday afternoon and/or evening, possibly bringing a period of heavy rain and lightning during the game.
Some models at this point indicate the storms may move through and be off to the north of Seattle by the start of the game (but would drench the pregame tailgating) while other models suggest the storms may hold off until later in the game.
It does not appear like it will be an hours-long event, more like an intense but passing period of heavy rain. But suffice to say, perhaps start planning on bringing some rain gear to the game, even through the day will likely begin sunny and warm. Also, the temperature will cool rapidly in the rain's wake.
Hopefully the rain won't dampen the enthusiasm. I seem to recall it rained quite a bit the last time the 49ers were in town for a night game and the Seahawks managed through that just fine (ahem, 42-13.)