Zermatt starts today at 7 °C and climbs to around 21 °C by the afternoon. That is a 14-degree swing in one day. The forecast stays dry in Zermatt and Chamonix until Saturday, when highs in Zermatt drop to around 15 °C. Start in a fleece, pack it away by noon. You will want it back the moment the sun leaves the valley.
Fleeces ↗