My partner and I hope to drive from San Francisco to Death Valley via Yosemite during the last week of April. I should like to find out whether Highway 120 is likely to be passable without the need for chains to be carried. Suggestions for an alternative route would be also be much appreciated.

Snow storms during the last week of April are rare. You should be fine.

Yeah, the temperatures can actually be fairly decent at that time of year, so you should be ok. You'd be very unlucky to get caught out, put it that way.