Here’s a real life example, setting out my thought processes as I worked through my suggested links to assess the prospects for 26th June 2017, followed by an account of how things actually turned out.
On this synoptic chart, I like the weak high pressure and reasonable spacing between the isobars, but not the cold front lurking to the north. I’m concerned that if that pushes in earlier than expected, high cirrus could shut down solar heating, or valley winds could suddenly pep up.
I’m pleased with this SRF forecast (the term “spring cloud formation” means cumulus development). It’s showing a great lapse rate, and the expectation of the first local showers from mid-afternoon with storms in the evening doesn’t bother me at all; this is typical of good days in the flying season. The prediction of cloud appearing in the southwest towards evening inclines me towards a one-way trip to the east rather than a triangle heading towards the Rhône valley in the afternoon.
The SRF chart confirms the verbal forecast………….
………….and their details for Eggishorn (Durschnittl means the average values, and Böenspitzen the gusts) suggest more wind early in the day than I’d like to see, but not that it will be too breezy for safe flying.
Meteoblue (as usual) shows lighter winds than the SRF prediction, but I hope the storms won’t kick off as early as suggested here.
The pressure difference chart causes me some concern. The peak value is below my rule-of-thumb limit of 4 hPa, but a very steep change from north to south overpressure, as shown here, can be associated with the sudden onset of föhn or other unexpected winds.
The Flugbasis forecast for winds at 3000m at 2pm looks good to the east of Fiesch, but that yellow streak on the far left (the Rhône valley) reinforces my initial feeling that a flight over the passes towards Chur would be a much more enjoyable prospect than a triangle involving a headwind battle to the west.
However, the corresponding chart for 2000m chart suggests some south wind pushing through from the Nufenenpass and Gotthardpass, which could produce dangerous conditions at those points.
At 7am, this chart of the actual winds rings an alarm bell – the blue arrow pointing at the “e” of “Schweiz” is the wind at the Grimsel Hospiz. I click on it for more details:
I’m not very happy with this wind strength, but I note that it is forecast to drop to an acceptable level by mid-morning, the earliest that I’d be able to arrive here. As I’m considering a flight to the east, I also check out the values for Gütsch, above Andermatt:
These values are also a concern, but there is a substantial funnelling effect at the location of the anemometer here, often resulting in much higher readings than the true wind speed away from terrain, so I consider that being able to stay upwind in this area without difficulty should be a reasonable prospect.
The emagramme suggests that there could be a stable layer around 3000m, leading to a slow start to the flight, but is otherwise favourable.
This looks like a good flying day to me, but it’s certainly not risk-free. The steep change in the north/south pressure difference could produce föhn or other troublesome winds, which may not be obvious from high altitude. Southerly winds could be funnelled through in the vicinities of the Nufenen/Grimselpass, and of the Gotthardpass (i.e. around Andermatt). I’m less concerned about the risk of overdevelopment, as this is easier to identify whilst airborne than are valley winds. The forecast of cloud in the southwest and a brisk west wind at 3000m in the Rhône valley motivates me towards a one-way trip over the passes to the east, rather than a triangle.
Here are the details of my flight:
I arrived at the Heimat takeoff to find a cloudless sky, but regular thermic cycles already established, and took off shortly before 10am. The climbs were weak and bitty, petering out at an inversion around 2700m (as one would expect from the emagramme), so it wasn’t until three quarters of an hour later that I felt I had enough height to cross to Bellwald. Only a single small cloud was visible ahead, over the 3500m Galmihorn, half way up the Goms, showing an obvious effect of wind from the south (see the header image on this page). There was a southwesterly breeze of around 10 km/hr along the Goms, and the thermals remained fragmented, with an apparent inversion around 3000m (still consistent with the emagramme). When I reached the Sidelhorn, it appeared that (as expected) the Grimselschlange was absent, as there were only light ripples on the Totesee (the lake in the col of the Grimselpass). The lazy rotation of the wind turbines near the Nufenenpass (in an anticlockwise direction from my point of view), showed no evidence of a breakthrough of excessive southerly airflow. It was an easy decision to continue to the east rather than to turn around and push back against a headwind which would be likely to strengthen the farther I progressed.
I succeeded in breaking through the inversion to reach cloudbase at 3500m, and found that my drift at this altitude was never more than 20 km/hr from the south, within my comfort zone. The air was buoyant and it was easy to progress over the Furkapass and then to continue along the Urserntal. Approaching Andermatt, I could see that the speed of rotation of the wind turbines at Gütsch indicated a brisk southerly breeze there, although I was experiencing only a gentle tailwind of around 10 km/hr from the southwest. I stayed well upwind and was able to reach the sunny slopes before the Oberalppass without any drama, where there was very little drift, and I connected with a smooth 5 m/s thermal to take me back up to 3500m and into the Surselva with plenty of ground clearance.
The next 20 km were straightforward, with small scattered cumulus marking good climbs and a 10-20 km/hr drift from the south-southwest, but I could see no further clouds beyond Brigels on either side of the valley. Turning around into the wind was not an atttractive prospect, so I decided to press on in the hope of usable lift ahead, but found only wind drifted scraps, and ended up landing by the station at Castrisch in a light westerly (although the normal valley breeze there would be from the east), having clocked up just over 100 km in under five hours.
Conditions had been much as expected, apart from a dearth of clouds when I had anticipated concern about overdevelopment!