I was there for 5 days in mid May last year and except for the last half day there were winter storms. Heavy rain in the valley off and on, even a little sleet and snow but no accumulation in the valley. And they closed Glacier Point road the day I arrived. It stayed closed for more than a week. Part of the time chains were required on the road from the south entrance into the valley (an issue if staying at Bass Lake). So I guess you have to be prepared for anything. That said, the waterfalls were amazing because last year was a heavy snowfall year. And I did get some cool photos. I just didn't get to do the hiking I had planned. And despite the weather it was still crowded. It will only get more crowded from this point through the end of summer. Doesn't matter, I'm going back again this year, just a couple of weeks later.