Massive Speculation: We might get RAIN in TS4 Cats & Dogs!

It's a neat theory. Nothing would truly surprise me after the way they dumped toddlers on us without warning. But then, apparently there were sprinkles of indications of toddlers in the game code prior to that, they'd just been dismissed as being way incomplete or having been there since near the beginning.

And I'm not sure there's been any indications of weather in the game code (least, not that I've heard of or seen myself). So unless they're just hiding it really well, probably not likely.