Mountain weather is very complex.
It is not common, statistically, to start a multi-day tour in a stable high pressure situation. It is often necessary to plan for one or more days of mixed weather and therefore provide the appropriate equipment. Weather forecasts are only reliable in the short term or in stable high pressure situations.
In any case, it is important to monitor the weather in the days leading up to the tour and to make a decision shortly before the start, as well as observe the weather throughout the tour.
For a good knowledge of the weather: MeteoSwiss