The details here apply to the conditions to be expected if you are reaching this point at a typical time on an eastward XC flight from Fiesch, i.e. late morning or early afternoon, before the north-facing side of the valley has started working.  From mid-afternoon onwards, you are more likely to be choosing your route based on what the clouds are telling you about the location of lift, which is much more variable then than earlier.

It’s better to consider the Grimsel and Furka region as a critical section of your flight, rather than simply a couple of obstacles to cross, as you may well fail to find lift on the other sides if your goal is simply to scrape over each pass, without plenty of ground clearance. I once asked one of the sky gods how he usually tackled this area. “Early and high” came the succinct reply! Indeed, if you are lucky enough to arrive there on a classic day with a 4,000m cloudbase, it may not even present a serious obstacle to your progress. However, even if you have been speeding along the Goms in abundant lift, it may well become scanty here, and promising clouds seem to have a habit of dissipating just as you reach them.  This effect – which I believe to be due to the Grimseler suppressing thermic activity – is increased with north overpressure. There is also the catch-22 that negotiating this area usually becomes easier during the course of the day, as base rises and thermals improve, but the later you arrive here, the less time is available to maximise the potential of the day.

In the diagram above, the four large red arrows on the left show the locations of the most frequently used thermals. There is often some weak lift running up the slope immediately to the north of Gletsch, where the Grimselstrasse (road over the Grimsel Pass) zigzags, but unless the Grimseler is absent or very weak, this may well be a nasty lee.  In south-westerly conditions, dynamic lift can usually be found at Serze below a good thermal which releases there, so this slope can provide a valuable option if you get low.  The Tällistock route is used much less frequently, most often if the Grimseler renders the Grimselstrasse area out-of-bounds, and tends to work better later in the day.  The thermal arising to the east of Gletsch is only a low save option, on which you cannot rely.

It’s very important to determine the strength of the Grimseler as you near the head of the Goms.  Its first manifestation may be a drift on the thermals from the north, or an increasing headwind.  I usually glance across at the wind turbines near the Nufenen Pass to the south to assess the airflow there.  Ripples on the Totesee (the lake in the col of the Grimsel Pass) and flags in this area may provide more clues.  Finally, if there are any gliders ahead of you, observing their behaviour will be very helpful.

The usual route here involves achieving plenty of height near the Sidelhorn (the last peak in the Goms), gliding over the col of the Grimsel Pass to connect with a thermal to the north of Gletsch, climbing high again, then heading over the Rhône Glacier towards the Klein Furkahorn, where you will usually be able to top up as you cross into the Urserntal there.  With cumulus development at the Sidelhorn, I would not leave on my transition until I was at base; in blue conditions I like to have at least 3300m, and often get overtaken by more confident pilots as I spend time hunting around in broken lift to maximise height before setting off.  But I prefer to have a delay here rather than to risk struggling around near terrain in an area with potentially tricky winds, and several times this tactic has enabled me to cruise high over a scratching gaggle which had left me behind a few minutes earlier due to my conservative attitude.

Here’s a typical example of a standard route through this area in good conditions:

If you set out from the Sidelhorn with less than 3,000m, you will have little height in reserve to allow for searching for your next thermal and must find it quickly. Confident pilots seem to favour the westerly facing slope north-east of the Grimsel col, rather than the south-southeasterly face overlooking Gletsch, but I dislike this choice, as I can see no easy landing options if that fails to work. I have often found a good thermal tracking up and releasing from the shoulder immediately to the west of the Rhône Glacier, which has provided me with a low save a couple of times, once from as low as 100m above the valley floor. I recommend achieving at least 3,200m before you start your transition across the Rhône Glacier, unless you are really struggling and have good reason to believe that the south-westerly slope above the Furkastrasse (road leading to the Furka Pass) is working well (e.g. a soarable breeze or gliders climbing there). Once again, with cumulus here, I would want to get to base before setting off, especially as there is often an easterly airflow blowing through the Furka Pass, and the lower you get, the stronger and rougher this headwind will be.

If you lose your height, it may be possible to get back up again in slope thermals between the Grimsel col and Rhône Glacier, but this area can be turbulent due to mixing air. I have seen the track log of a pilot caught in the Grimseler here, who flew directly downwind towards the north-westerly facing slope above the entrance of the Furka train tunnel, to the east of Oberwald, and pinged out from there, but have not tried this manoeuvre myself. As long as the Grimseler is not too strong, it is usual to find a gentle katabatic flow from the east off the Rhône Glacier if you go down in Gletsch, but sometimes utterly chaotic conditions rule there. When the Grimseler is held back due to a south-westerly flow, an alternative recovery option is to press forward to Serze, as shown in this track log:-

Although I had been unable to centre the wind-drifting thermals above Gletsch, I found enough dynamic lift at Serze to gradually gain height before connecting with a good climb which produced enough altitude to allow me to cross directly into the Urserntal.

Here’s another example of a less than ideal route, which very nearly decked me in Gletsch:

After an overconfident line directly towards the Klein Furkahorn under a line of forming cloud, expecting assistance from a SW airflow, I found the katabatic wind from the Rhône Glacier dominating the scene at the Belvedere.  Luckily, I had enough height to be able to beat a hasty retreat to the usual thermal running up the shoulder just west of the glacier, which hoofed me straight up to base and allowed me to continue the flight.

My final example arises from a catalogue of errors, resulting in an extraordinary tour of the area:-

This route was only possible because of a south-westerly wind blocking the Grimseler. However, even when I had finally got up, and had 3300m as I started my glide towards the Klein Furkahorn, an easterly wind blowing through the col of the Furka Pass meant that I barely squeaked over, though once I was across, it provided some dynamic lift to enable me to bounce along to Realp where I could then pick up a decent thermal.

Although soaring up the slope next to the Furka Pass may work in a south-westerly, this won’t (on its own) provide enough height to cross over with a good chance of getting up on the other side. It’s much better – if possible – to ping out in a thermal tracking up the spine above the Belvedere hotel and releasing from the Klein Furkahorn. If you find yourself slope soaring here, it’s best to be patient and to try to work towards the higher slopes to the north, away from the col, to maximise your height rather than to scrape over as soon as it appears possible. The other side of this ridge may be in the lee in such conditions; also, if you fail to find a thermal there quickly, you might find that you have insufficient height to be able to glide out to Realp, down in the valley, resulting in a tricky landing in the boonies.

Before you cross over into the Urserntal, I suggest you identify the position of what seems to be the most reliable (and obvious!) thermal trigger on the far side: the Chli Bielenhorn, a small rocky peak about 2½km to the east of the dividing ridge, as shown in the picture below.

With a modest top-up off the Klein Furkahorn, you can expect to be able to glide straight to the Chli Bielenhorn for your next climb.