When we have been there at that time (late June/early July), we have encountered everything from cool/cloudy/rainy to warm and sunny, often on successive days, or even on the same day. In Zermatt, we hiked up to Riffelberg in snow (light snowfall, not a blizzard, but it was cold). From there we took the train up to Gornergrat, where the sun was shining but it was still very cold. The next day, July 3, dawned bright, warm and sunny.
Before Zermatt, we were on Rigi, where we had one cold foggy day and one of glorious sunshine, and then Wengen and Mürren, where again it alternated between cool and drizzly (not hard rain) and nice sunny weather.
Heading to Montreux from Zermatt (now around July 8), we found it hot and humid---almost unpleasantly so.
So we just plan to dress in layers---light shirts for war weather, with a microfleece jacket or pullover and a good rain jacket for adverse weather.