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 Lugano (to the south) and Zürich (to the north) is available at Profiwetter:

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 Wind und Wetter.

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 Binn, Ulrichen, Visp, and Altdorf; a significant easterly in the first three or southerly in Altdorf rings alarm bells.  I have been told that a wind speed at Binn of 30km/hr is considered the upper limit of safety.  Checking the situation at Gütsch is said 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.  Near 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.

If you decide whilst flying that föhn has broken through at valley level, but landing high does not appear to be a reasonable option, then you should try to avoid locations exposed to wind from the side valleys mentioned above.  The Fiesch landing field is not too vulnerable to the wind from the Binntal, but Lax (safer if Bise has broken through) can be horrendous in that situation.  If you are near Ulrichen and can reach Ritzingen, which is downwind, that would probably be your best option; you should not overfly the Münster sailplane runway low, but if it appears deserted, prioritising your safety may appeal.  If that’s out of range, Oberwald would be a reasonable alternative ONLY if you can see evidence (e.g. another glider landing there with good groundspeed) that the Grimseler is absent (which may well be the case in south overpressure).  Near Brig, Bitsch is less exposed to the wind blowing down from the Simplon pass than options further west.  In the Urserntal, Zumdorf (half way between Realp and Hospental, identifiable from the air from the gravel works) is probably your best bet, and near Disentis, a downwind dash to Trun is likely to be safer than landing there in föhn.

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, including various charts, illustrates some of these points.

I hesitate to include an example flight here, as flying with föhn around is not to be recommended.  This one took place in borderline south overpressure of 4 hPa, which certainly resulted in föhn in Andermatt at the time I overflew the village, and led to some apprehension about landing conditions, but caused me no problems in the end: