Fiesch is vulnerable to föhn in airflows from the north as well as the south, but most Swiss pilots use the term to refer to a condition which arises in southerly regimes in this location, so for the purposes of explanation, I have adopted that convention here.
Although getting caught in full-on föhn in these big mountains would be unpleasant and dangerous, a forecast of “mild föhn tendency” is often associated with excellent flying conditions, particularly for long one-way XCs to the east, so you may find that you are having to balance this risk against the potential for a good flight.
The conventional approach to the prediction of föhn is based on the forecast of the difference in atmospheric pressure in the north/south axis across the Swiss Alps. A chart of the pressure difference between Zürich and Lugano is available at Meteocentrale.
It’s said that the risk is significant if the value exceeds 4 hPa. In my experience, it really isn’t as simple as that; in the spring, 2 hPa can be enough, and higher humidity (or precipitation) on the side of the Alps from which the wind originates can also increase its likelihood. (South) föhn can even arise in Andermatt when the weather data is showing north overpressure; although this is not unusual with an approaching cold front, it can also occur with no obvious explanation. I have often noticed that a rapid change of pressure in either direction is associated with unexpected winds, not always because that indicates some general weather disturbance around. No sensible pilot launches in Fiesch without a rough idea of the pressure difference, and I have even seen some using their smartphones to ascertain the current value of this parameter as part of their final preflight check. A chart showing differences between a variety of locations (and hence axes other than north/south) can be found at Flugbasis.
Independent of the pressure difference (but often associated with it), an approaching Atlantic depression and cold front usually guarantees föhn, which can often start to develop even when the synoptic chart is showing that the front is still located in the western half of France.
Local tandem pilots often use their smartphones in the cabin on the way to takeoff to check out observations of the wind speed and direction in Ulrichen, Visp, and Altdorf; a significant easterly in the first two or southerly in Altdorf rings alarm bells. An observation station at Binn (in the side valley to the immediate south-east of Fiesch) can be found on Spotair; I have been told that a wind speed there of 30km/hr is considered the upper limit of safety. Checking the situation at Gütsch may be helpful, but having spent a lot of time in Andermatt experiencing what actually happens, I am unimpressed with its reliability as a predictor of föhn. The conditions at valley level are often quiet even with a substantial southerly flow up there, and I have occasionally observed föhn blowing down the Gotthardpass when there’s a light northerly at this location. I believe that the reason for the erratic correlation is that the weather station is at 2287m, whereas föhn may develop as a valley phenomenon in advance of its effects at higher altitudes.
From takeoff, the typical signs of föhn – a milky sky, lenticular clouds, and cloud spilling down from behind the mountains to the south, are easy to spot. If you are airborne around Fiesch and hazy conditions suddenly clear when föhn has been predicted for later, this is a sign that it may already have broken through down in the valley and suggests that you have outstayed your welcome. The safest option in this situation may be to land at takeoff level.
Föhn can be manifested as a turbulent wind from any of the side valleys which lead in to the main valley from the south. Around Fiesch itself, this will be from the Binntal (on the right in the picture above). In the upper part of the Goms, it arrives in Ulrichen from the Agenetal, turning to flow down towards Fiesch, but may also cause the thermals on the south-facing side of the valley around this area to become turbulent with a substantial upslope drift. It blows from the Simplonpass into Brig, and from the Vispertal at Visp, causing a reversal of the usual westerly flow in the main Rhône valley. Around Andermatt, it appears from the Gotthardpass, and at Disentis from the Lukmanierpass via the Val Medel, with the potential to produce significant turbulence not only by itself but also by conflicting with the usual valley flows, so if you have to land around either town when föhn has broken through there, it would be better to try to find an option well above the valley floor.
Finally, another curious anomaly to bear in mind is that weak developing föhn is often associated with a breeze from the north-east in the Fiesch landing field, which seems completely paradoxical – but it happens!
A case study, comprising various charts, illustrates some of these points.