Synopsis:
The Pyrenees remain between two cut-off lows, one to the east over Italy – slowly filling and with decreasing influence, and one to the Portuguese Atlantic coast, increasing it’s influence over the region. Warm and increasingly moist air from the south is advected into Spain, leading to increased cloudiness and instability.
How strong this advection is and how far it extends into the Pyrenees depends strongly on the position of the Portuguese cut-off low, which is subject to considerable uncertainty.
Area around TP2 (Pyrenees Atlantique):
During the night and early morning, some high level clouds will cross the Pyrenees. They can hinder the solar radiation, but the radiation should be enough to develop thermals in the course of the morning.
The thermal forecast looks similar to today: rather stable conditions in the lower levels, but usable thermals developing just before noon. Rather weak NNE winds should prevail in the thermal layer, while the top of the thermals will reach around 1600-2000 MASL west of TP2 and up to 3000 MASL towards Pic du Midi.
The NNE easterly valley winds will reach around 1500 MASL, above which moderate S to SW flow will prevail (the strength of the wind depends on the large-scale situation and is modelled in a range between 10 and 25 km/h at 2500 MASL).
There will be some overdevelopment, when and where it will start first depends on the model you’re looking at. Likely the area around TP2 will trigger the first showers on the French side.
Spain (Area of Castejon de Sos and towards Catalonia):
Also in the Spanish Pyrenees, some clouds will move through the sky during the early morning and forenoon.
Nevertheless, thermals should develop during the forenoon, reaching peaks around 3000-3500 MASL in the high Pyrenees and around 2600 MASL in the foothills. The lower layers in the foothills are more stable and the thermals will start later, but once developed they’ll offer good climbing rates.Southerly to south-westerly winds of 15 to 25 km/h will prevail, with the possibility of a light WNW tailwind above 2500 MASL.
The likelihood of overdevelopment along the route is also real in Spain, but the airmass is a little drier towards the east, so conditions look a little better towards TP5. Overdevelopment in the eastern parts of the Pyrenees seems to be mainly on the French side of the range and the areas towards the Mediterranean where the airmass offers more moisture.
Thermal forecast for the Spanish Central Pyrenees from XCtherm.com
Outlook:
With the Portuguese cut-off low moving in slowly, the outlook remains very uncertain. Some models show slowly deteriorating conditions with increasing cloudiness and storminess from day to day, while others show decent conditions until midday on Friday.
What does seem certain is that the remnants of the cut-off will pass over the Pyrenees on Saturday, leading to widespread thunderstorms and rain on the final day of the race.
