Chamonix takes the brunt today, with around 32 millimetres of rain before the front clears. Then it settles. Two dry days climbing towards 22 degrees. The catch is the nights: Zermatt drops to 6 degrees after dark, a wide swing from the afternoon warmth. Pack for both ends. A merino base layer covers the cold dawn without weighing down the midday climb.
Merino base layers ↗