Föhn case study

I decided not to go to Fiesch on 13th October 2018, as the weather forecasts indicated too high a risk of föhn as far as I was concerned.  These are the charts showing the information on which I based that decision, and how things actually turned out:-

UK Met Office synoptic chart for 12:00 UTC

That degree of spacing between the isobars would normally be associated with a general meteo wind of noticeable but not uncomfortable strength in the high mountains. However, the pair of cold fronts approaching from the Atlantic rings an alarm bell, even though the chart shows them as not having even reached the French coast by the middle of the day.

Meteocentrale Zürich-Lugano pressure difference chart

The north-south pressure difference chart shows borderline values for 13.10.18 – an increase from 2 to 4 hPa over the course of the flyable part of the day.

Flugbasis chart of winds at 3000m at 13:00

Flugbasis predicts a very light southerly at 3000m in the Goms, albeit with some funnelling through the Grimsel pass (no Grimselschlange!).  The easterly in the main Rhône valley may only be around 13km/hr, but it’s blowing the “wrong” way (i.e. downwards).

Flugbasis chart of winds at 2000m at 13:00

However, the chart for 2000m is ugly! Although the Goms looks ok, winds around 24km/hr to the south-east of Fiesch, at Oberwald, and west of Brig have the potential to generate nasty conditions in those locations, and Andermatt could be quite malignant.

So how did things turn out? Here are the charts (from SpotAir) for key weather stations, showing the actual values at 17:00, with the history over the preceding hours:

The key points here are

  • the föhn eventually broke through everywhere
  • it appeared at quite different times, for no obvious reason
  • the onset was (in most locations) relatively abrupt compared with “normal” valley winds

I was content with my decision not to go to Fiesch that day.