Hi Quirky -
<<"Thanks also for the weather report! Didn't realise how cold it could be in May. ">>
Please note that I mentioned altitude.
I've been up at Männlichen ( above Wengen at about 2230 meters, or Kleine Scheidegg, near the Jungfrau, at about 2060 meters in early June and found the trail between the blocked by snow a few meters deep in the shaded parts of the trail. Although that particular trail is usually not open until about June 15th, it can be earlier or later than that, depending on the snow pack from the previous Winter.
On the other hand, at low altitude....say Luzern, Montreux, Bern, May is definitely Spring.
I forgot to put this next link in my last post. Links on this page will give you more details about weather, etc.
In particular -
April can be the start of Spring at lower altitudes. it is often rainy, and the weather is very changeable.
In 2014, in the middle of April there was a very unusual stretch of warm sunny days over most of Switzeland, lasting about a week.
I was in the Emmental, above Sumiswald, at a "Lueg" ( Swiss German for viewpoint or scenic viewpoint) with wonderful views of the alps, if the weather is clear. It was. I was at about 800 meters altitude on the hillside; Sumiswald is at about 680 meters.
(By the way, if you need a detailed map of Switzerland which shows altitudes and a whole lot more stuff, try this one. it has more information for professionals buried in it than the map I linked you to earlier. I use it when I want altitudes and hiking trails.
map.geo.admin.ch/?topic=ech&lang=en&bgLa yer=ch.swisstopo.pixel karte-farbe&layers=ch.bav.ha ltestellen-oev,ch.swisstopo.swiss tlm3d-wanderwege&layers_visi bility=false,false&E=2 668410.66&N=1211047.00 &zoom=4
I have set it for Luzern. The search function works well for place names, for example.)
I was able to get a series of photographs which I combined into the attached panorama. Note that on about April 15th, the lowest area were just barely becoming green, the trees were blossoming but are only starting to have their leaves, and there was a lot of snow left on the Alps. Spring was coming.
Usually, you have to wait for may to get that much good weather. I think you have picked a good time for your trip. The later in May, the better. But, even by the 8th, there will be plenty of greenery. And, the lower altitudes in the Alps (such as Lauterbrunnen) will be pretty much free of snow.