Today's showers clear overnight. Thursday and Friday come up dry in both Chamonix and Zermatt, with valley highs of 18 and 17 degrees. The mornings stay cold: 10 degrees in Chamonix, under 8 in Zermatt, and colder again as soon as you gain height. That gap between dawn and afternoon is the whole trick of September. Pack for both. A warm layer you can stow by nine, and reach for again on the descent, beats a midlayer you carry all day.
Merino base layers ↗