Living in Mountain View and driving with Google's self driving cars every day, I have to say I think they are quite a bit further than you give them credit and those conditions appear to be less limited than you are letting on here.
They do well in rain but snow remains a problem, not due to icy roads (at SXSW Chris Urmson said they do well on slick roads) but that the "better than GPS" LIDAR-based location tracking stops working when snow is on the ground.
The car simply cannot figure out where it is when there's a significant layer of snow on the ground because the reference maps stop looking like what the car is seeing.
