Although the day/night cycle is fairly realistic changing between day and night at 6am and 6pm Eorzea time (which is around the time of day in the real world dawn and dusk will occur, barring seasonal variations of course), the weather system not being random and instead changes at fixed times doesn't seem realistic and appears to just be a placeholder that was overlooked during development and never changed.

It's not a game-breaking issue, but does become noticeable if you stand in the one zone for an extended period and see the weather change like clockwork, reminding me of that joke from Back to the Future II when Doc looks to the sky and tells Marty exactly when it will stop raining: "Right on the tick! If only the Postal Service was as reliable as the Weather Service!"