Chamonix should reach 19 °C today under mostly clear skies, and Wednesday stays largely dry. Thursday does not. Forecasts show around 33 mm of rain in the valley, with temperatures dropping to 5 °C. Zermatt expects 10 mm and a low near zero. Plan longer routes for today or tomorrow. If Thursday is your only day out, pack a fully waterproof shell and a dry layer for the descent.
Rain jackets and ponchos ↗