Meteo Day 3 (25.6.): The sandwich

Synopsis:

The Pyrenees lie between a slowly filling, but quasi-stationary, low pressure area over the Mediterranean and a forming low pressure area to the west of Spain. In between, a narrow ridge extends from the western Pyrenees to the North Sea. With the ridge still present, an inversion between 2000 and 2500 MASL is still present, but much weaker than on day 2. Meanwhile, the forming cut-off to the west leads to advection of significantly warmer air into Spain and the Pyrenees at lower levels.
At first glance, the changes on a large scale are small, but in detail the conditions will change significantly between day 2 and day 3, with the wind turning almost 180 degrees in some cases.

French Basque-Country / Pyrenees Atlantique (TP1-TP2):
At least the humidity doesn’t seem to be a big problem anymore – the weather should stay the same all day. However, the wind situation is changing significantly. In the early morning, a strong southerly wind (25-50 km/h) will arrive above 1000 MASL. The warm air associated with this southerly wind will make it very difficult for thermal activity to develop, so the whole of the morning will see fairly stable flying conditions in the lowest altitudes down to about 1200 MASL. Those who are able to take off above the overnight stable layer will see the thermals develop much earlier, but will also have to contend with the rather strong and potentially turbulent southerly wind. This effect is more pronounced the closer you are to the Spanish border.

Once a thermal boundary layer is established in the French valley, it is likely to remain capped around 2000 MASL (probably lower) and moderate (10-25 km/h) valley winds (mostly NNE) will prevail at these altitudes.Due to dynamic effects (decreasing pressure in the Lee of the Pyrenean main chain), the valley winds will be stronger than the – relatively weak – thermal activity would suggest.
Above this cap, we have significant wind shear and the persistent moderate to strong southerly wind.

The French side of the area between TP2 and the Vallee d’Aure (around Pic du Midi) appears to be very complicated with two stacked airmasses; the lower, capped and relatively stable French airmass with northeasterly winds up to about 1500 MASL and the upper, airmass with better thermals but windy conditions due to the southerly wind.However, the southerly wind is modelled to be less strong in this area than to the west of TP2.

Interestingly, above about 3500 MASL, a northerly wind prevails (hence the title “The Sandwich”).

Spanish Pyrenees (Panticosa – Monte Perdido – Boi Taull):

The good news:
Once in Spain, there’s only one air mass to keep you busy. Moderate (15-25 km/h) southerly winds dominate the thermal layer.
The advection of warmer air also delays the start of the thermals in Spain, but there should be developed thermal activity around midday, with good climb rates and a base around 2500, probably up to 3000 MASL.
With these southerly winds, flying conditions will also be quite difficult, but it’s likely to be a little easier and less complicated than on the French side.
However, there seems to be some risk of thunderstorms and showers towards the evening, especially east of the Castejon de Sos area.

Summary:

It looks like we are in for another complicated day of flying. Different layers of air masses will dominate Tuesday in France: A lower, rather stable layer up to about 1500 MASL with winds from the NNE, a layer with rather strong southerly winds between 1500 and 3000 MASL and a third layer with northwesterly winds above (the last one is rather insignificant for the race). Thermals will develop quite late in the lowest layer.
In Spain, the mountains remain entirely in the middle layer, offering a late start and windy conditions, but with rather good climbing rates.

Outlook:
Wednesday and Thursday are a bit uncertain as it’s really hard to predict what the cut-off to the west of Spain will actually do. At the moment it seems to be less windy, but increasingly stormy.