I happen to live in California, and while autonomous vehicles will have to deal with mountains eventually, that's not their primary target. If I want to see snow, the mountains are a couple hours away. Chains or 4WD/AWD are required. I would guess this will be one of the last holdouts for human drivers. However, they don't get buried under multiple feet of lake effect snow over and over again throughout the course of a winter, which was really the problem I was thinking of -- and a problem that must be tackled if these cars are to see any success in Michigan, northeast Illinois, northern Indiana and Ohio, upstate New York, and southern Ontario. It's not a substantial problem here, as we lack massive bodies of fresh water for systems to tap into, freeze, and dump on the land.
I also said "sunny days in California don't expose the hazards", and there are many of them. While the other conditions certainly exist, they aren't as relentless even if they can be as episodically intense. That's part of why people pay so damn much to live here!