How late in September (generally) before weather turns?

Never any guarantees, but late Sept and early Oct are normally fine for hiking. There are the odd snow showers that may make things challenging for a day or two and sometimes you get stuck with rainy days, but just as often you get perfect sunny fall weather.

This year we were in the Berner Oberland (Gimmelwald, Stechelberg, Wengen) hiking in t-shirts on Oct 1st and 2nd!! Previous years we have always done some hiking well into October. Consistently wet/cold weather usually doesn't set in until late Oct/November - and last year, we were even up at Harder Kulm in late November, eating outside and hiking in sunshine (winter hats and jackets needed but no snow or wet trails).

In 2019, the weather was excellent for hiking from September 24 – 30 in the Berner Oberland region. It rained one afternoon, and the next day was cloudy, but there were several spectacularly sunny days as well.

All the best,

Raymond

You can find actual, day-by-day, historical weather data on the website timeanddate.com. It goes back about ten years so you can (and certainly should) check multiple years. Unforturnately, I just see temperatures for the two Berner Oberland locations I checked; most other locations also have a precipitation indicator.

Here's Lauterbrunnen for September 2021: https://www.timeanddate.com/weather/@2659992/historic?month=9&year=2021

And here's Murren for the same month: https://www.timeanddate.com/weather/@2659530/historic?month=9&year=2021