No rain forecast for Chamonix or Zermatt until Saturday, with highs near 20 degrees. The catch is the start. Zermatt sits at 8.8 degrees at dawn and Chamonix at 12, so the first hour feels like November and the last like summer. Pack a thin merino layer you can stuff away by mid-morning rather than a jacket you will carry all afternoon.
Merino base layers ↗