The fact that the game is in los santos should be enough to tell you there won't be any snow in the streets. None, anywhere aside from the mountains, none. At least I hope not. That would be a huge fail on the devs part. Come on people!
It IS a video game, y'know? They can put snow wherever.
Snowing in a city? No, not quite that. Snowing a couple blocks from an analog of Hollywood Blvd? That's a bit far-fetched. Especially, as I mentioned, the drop-top and the guy in the tank top. If you played Bully, you'll know that R* can and will give NPCs weather appropriate attire with changing seasons. Based on this, we can reasonably assume that no, it is not snow you're seeing.
"Star bringing purple! Star bringing yorbel! STOP BRINGING YORBEL! STOP RINGING YOUR BELL!"