Many general European weather sites will provide information relevant to Switzerland.  Bear in mind that weather forecasting in mountains is notoriously difficult, and predictions often change radically from one day to the next.

You will probably have your own favourite synoptic charts, but as a Brit, I still favour the UKMO’s.  Those at  Weatheronline extend much further into the future, but I would only use these pages as starting points for forming a positive or negative opinion of the general prospects to be expected, and certainly not for trying to make accurate predictions for specific days more than a week ahead.  It may also be worth looking at a forecast of the jetstream, which has been blamed for creating stronger winds than one might have predicted from the surface pressure chart.

For more specific charts showing predictions of wind, cloud, and precipitation, I use XCTherm and Meteoblue.

Charts at Windy offer a vast range of options, both in terms of parameters and models, extending over the next six days.

There are plenty of sites showing basic forecasts to help you to estimate the chance of decent flying conditions over the next couple of weeks; I find Meteoblue’s 14 day chart for Fiesch as good as any, taken with their 7 day chart for Eggishorn.

The Swiss TV/Radio website provides a simple general overview of the weather situation over the next five days, and a link to the latest TV forecast.  In my experience, their general chart of Switzerland tends to predict much more cloud and precipitation than actually occurs, but the forecasts for specific locations, e.g. Eggishorn, showing a projection of conditions every three hours for forthcoming days, are more accurate.

I don’t know of any free websites which show reliable thermal forecasts for Fiesch.  In the summer I pay CHF 5.- per month for access to the Regtherm data via XCTherm, which includes a prediction of the XC potential of the next four days, based mainly on the expected thermal quality.

Daily and longer-term forecasts (in German), aimed at sailplane pilots, are often published by Flugplatz Schänis, located in Eastern Switzerland – close enough to be relevant to Fiesch.