Jul 26, 2016 - 6:46 PM
in reply to jsanderfield
It is not possible to answer your question for the reasons you note.
However, typically September has the best weather of the year in the area around Lauterbrunnen. The typical pattern is sun in the morning, clouds beginning to gather by early afternoon, then a late afternoon quick thunderstorm. We have been to the area for 19 years in a row during September and only once (in 2001) did the cool and rainy fall weather settle in during early September.
Be aware that weather forecasts in the area are really only accurate up to about 36 - 48 hours in advance. That is because the mountains make their own weather.
If you want to wait to the last minute (and I do mean last minute!) you should have no problem finding lodging in September. That is if you can be flexible. If you want a particular hotel you should book in advance to prevent disappointment.