Chamonix and Zermatt both sit under clear skies with no rain in the forecast to Friday. The catch is the spread: Zermatt starts the morning at 8C and climbs past 20C, Chamonix runs 11C to 20C. That is a jumper's worth of difference between the car park and lunch. Pack for both ends. A merino base layer under a light shell beats carrying a midlayer you never put on.
Merino base layers ↗