In mid-May, in an average year, I wouldn't plan on hiking anywhere above 1800 metres in most of the country if you want to avoid snow. As mentioned above, if you head down to Ticino then this may go up by a couple of hundred metres.

It may pay to wait until closer to the time to plan the exact location - by late April we'll know how much snow fell during the winter and so how low you'll need to keep in order to avoid it. At the moment, it's looking pretty good for a lot of snow this year, but that could easily change.

Alternatively, come prepared with some gaiters and go tramping though the snow that you do find - it can be quite fun, and if the sun is beaming down on a warm Spring day then you won't be cold. I'm not recommending anything extreme, I'm just saying that snow shouldn't be avoided if it means not being able to enjoy the remainder of a mostly snow-free hike.