“I’ve heard that Fiesch is generally turbulent/windy/dangerous; is that right?” is the sort of question that I am asked most frequently by pilots who have never flown here. My answer is that hazardous conditions do not arise without a reason, which can nearly always be predicted in advance, enabling you to avoid trouble. However, as larger mountains are generally more sensitive to factors causing turbulence and wind than smaller ones, the occasional challenge to glider handling skills is an inevitable consequence of the topography which produces such exceptional flying here. Also, it’s common to encounter valley breezes around 30km/hr when landing out in mid-afternoon.
As far as generalised roughness is concerned, the time of year is an important consideration. In the spring, the combination of a steep lapse rate and intense solar heating can produce strong, sharp-edged thermals, as it can anywhere, but this feature subsides over the course of the season. At any time of year, winds of even moderate strength at altitude and/or significant pressure differences across the Alps can cause conditions which may be dangerous as well as unpleasant. Finally, it may occasionally be uncomfortably choppy due to an inversion, which is less easy to predict.
Localised turbulence is a separate issue. The wind in the main Rhône valley west of Brig usually becomes quite strong in the afternoon, often exceeding paraglider trim speed at ground level, so the further you go in that direction, the more likely you are to encounter challenging aerology, especially if you cannot stay airborne or at least keep well away from terrain. Mixing winds in the Grimsel/Furka Pass area and around Andermatt can also produce sudden disturbances in previously benign conditions, so I prefer to stay high in these regions, particularly when there is any significant airflow or pressure difference in the north-south axis.
Approaching weather fronts are particularly risky, in that they may cause days which start with pleasant flying conditions to turn nasty quite suddenly. In flatlands, it may be relatively easy to see weather disturbances in the distance, with the associated changes of wind strength and direction developing gradually enough for pilots to have time to land before conditions become unmanageable. In these large mountains, views may be blocked, and funnelling effects can cause wind speeds to change much more dramatically. In particular, approaching cold fronts can cause föhn to break through abruptly from any of the side valleys which lead in to the main valley from the south, e.g. down the Nufenenpass into the upper Goms, the Binntal near Fiesch, the Simplonpass at Brig, the Vispertal at Visp, the Gotthardpass into Andermatt, and the Val Medel at Disentis.
For further explanation of factors relevant to unpleasant conditions here, the Weather page addresses some of the basic principles, and the pages under the Winds heading set out more specific details. Meteo sites with the parameters enabling you to assess the actual risk on a particular day are listed in the Looking ahead and On the day pages in the Links section. In a nutshell: if you’re keen to avoid nasty conditions, don’t fly on days when the forecast shows a significant pressure difference across the Alps, winds of more than light strength, or approaching weather fronts!