Thursday brings up to 45 mm of rain to the Chamonix valley, with thunderstorms in the forecast. Daytime highs drop from 17 to 12 degrees. By Friday morning the valley floor cools to around 1 degree, so rain may turn to snow higher up. Leave exposed ridges and glacier routes for later. Zermatt gets off lighter, with under 6 mm on Thursday and a dry Friday. Pack a shell either way.
Technical shell jackets ↗