Maybe focus on what the weather will do to npcs instead? For instance, rain makes some npcs run to get out of the rain or cover their heads, reducing their vision? Maybe more npcs in areas of the map where they're sheltered from the rain and less npc in unsheltered areas? Npcs grouping up and "waiting out" the rain
If snow is an idea, maybe add hidden moving spots around the map where there's a small chance of slipping and falling (you can't cover yourself until you've stood back up)