Exclusive weather spawns
Yes, there are spawns which will be exclusive for a set weather condition. We don’t know which Pokemon are part of that spawn pool at this moment.
Increased weather spawns
You can recognize a weather specific spawn by a special “swirly effect” enclosing the Pokemon. Similar to how Incense and Lure spawns have a special marker around them, the weather specific spawns can easily be recognized by the swirly marker.
Weather spawn stats and CP
It has been confirmed that weather spawns will feature higher wild CP (capped at Level 30 for normal spawns) and it’s possible that they will feature increased IV values. We are unable to confirm or deny the later statement at this moment, but the official press release states the following:
The new dynamic weather gameplay will add unique ways for Trainers to engage with Pokémon based on real-world weather including changing Pokémon habitats, increasing Combat Power, and earning more Stardust.
More Info: pokemongohub.net