6 to 12 inches at pass level of snow and 2 feet at higher peaks in California

This actually makes me worry more about backpackers than people in cars. Often you can survive in a car for days with just your heater and a little water if you are snowed into your car somewhere. But, unless people took enough warm clothes and waterproof clothes and know how to survive this level of snow in the high country many could already be dead in these kinds of conditions along the John Muir Trail for example. But, if you don't know how to drive in snow (and many people do not in California by the way) you shouldn't really be out on roads with snow.

If you have a front wheel drive car by the way if you are going down hill often the rear end of the car is going to come around towards the front if you are downshifted into a lower gear. If you have a hand brake you you can apply some pressure with your hand brake and this should straighten out the car. But, if you only have a foot emergency brake this becomes much more problematic.

So, mostly unless you have a rear wheel drive car or an all wheel drive or 4 wheel drive car or truck (AND) you know how to drive in snow you probably shouldn't be driving at all if there is snow on the ground. (unless) you are going to be very careful in learning how to drive in snow.

If you have never driven in snow before: You usually don't ever apply your brakes full on unless you want to be sliding sideways soon. In other words your brakes aren't very helpful in snow. Unless you understand this you shouldn't be driving at all in snow. So, you have to learn to use your transmission to slow yourself down usually by downshifting slowly through your gears. This is usually possible in an automatic but likely you might need to study how to do this with your manual if you haven't done it before or talk to someone who knows how to do this already.

Also, if you have a two wheel drive car or vehicle those drive wheels are just going to spin if you apply too much power to them all at once like you are probably used to also.

So, once again if you are not used to driving in snow or ice you have a lot to learn not to destroy your car or hurt or kill yourself or others. Just be safe (especially in the Sierras) or at altitude anywhere in California now.