Météo France has placed several departments on yellow alert for violent winds on Wednesday November 20. These are Hautes-Pyrénées, Gers, Haute-Garonne, Ariège, Pyrénées-Atlantiques and Landes. Snow is also forecast in the mountains.
Météo France has placed six departments in the South-West on yellow alert this Wednesday, November 20, 2024. Hautes-Pyrénées, Gers, Haute-Garonne, Ariège, Pyrénées-Atlantiques and Landes are affected by violent winds. Wind gusts greater than 80/90 km/h are, in fact, forecast. Furthermore, a yellow wave-submersion alert is activated for two of them (Pyrénées-Atlantiques and Landes). The Pyrénées-Atlantiques department is also placed on yellow rain flood vigilance.
Yellow warning for violent winds: details by department
Hautes-Pyrénées and Haute-Garonne
Both departments are on yellow alert for strong winds from midnight to 6 p.m. Intense gusts could disrupt travel and outdoor activities.
Gers
The Gers department is affected by a yellow alert for violent winds from midnight to 7 a.m. Caution is advised when traveling in the morning.
Pyrénées-Atlantiques
The Pyrénées-Atlantiques will be on yellow alert for violent winds from midnight to 6 p.m. and for waves-submersion from midnight to 9 a.m. The department is also placed on yellow rain flood alert from 6 a.m. to midnight.
The Landes
The Landes are also on yellow alert for violent winds from midnight to 6 a.m., and on wave-submersion alert until 8 a.m. The Atlantic coast could experience rough seas, requiring the vigilance of local residents and users.
Ariège
Ariège faces significant wind gusts, with a yellow alert in effect from midnight to 10 p.m.
Tarn-et-Garonne
Tarn-et-Garonne is also placed on yellow alert for violent winds. It will have to face significant wind gusts from midnight to 6 a.m.
Snow in the mountains: be careful on the peaks
The disturbances forecast for this week are expected to bring snow to the Pyrenees. Two notable episodes are expected Tuesday-Wednesday and Friday. Precipitation, accompanied by a west to northwest flow, will result in:
- A rain-snow limit oscillating between 1800 and 2200 meters.
- Accumulations of up to 20 to 30 cm from 2500 meters, mainly on the western massifs of the Pyrenees (Pyrénées-Atlantiques and Hautes-Pyrénées).
- Gusts reaching 80 to 90 km/hmaking conditions at altitude particularly dangerous.
These precipitations should temporarily cover the summits with a nice layer of fresh snow, but the direction of the flows limits the significant snowfall to the entire range.
The weather this Wednesday
The active rainy disturbance from the day before slid over the south of the country with precipitation in the morning extending from the South-West to the Alps. The rain-snow limit in the Pyrenees is around 1500-1700 m. A few showers over the Massif Central and the Alps with snowflakes from 700-800m. Finally, snowflakes also fall on the Vosges from 300 m and the Jura around 400m. Behind the disturbance, the weather is calmer in the northern half with an alternation of clouds and clearings, more numerous from the Paris region to Hauts-de-France. In the afternoon, attenuating precipitation blocks over the Pyrenees with snow from 1500 m and some more snowfall in the form of showers over the Massif Central and the northern Alps. Further north, a pattern of showers sets in mainly from the northern coasts of Brittany and Normandy to Champagne via Ile-de-France, with locally some sleet possible and gusts reaching 50 at 70 km/h. The wind remains very sensitive in the south with gusts reaching 60 to 80 km/h south of the Garonne and up to 70-90 km/h and occasionally 100 km/h in the Tramontane region and the Alpine and Pyrenean ridges. . In the South-East, gusts reach 60-80 km/h on the Mediterranean coast, but it is in Corsica that a stormy and lasting wind is the most powerful with gusts up to 150-180 km/h on Cape Town, 110-130 km/h in the Bastia region and 120-150 or even more on the terrain and in Balagne throughout the day. The wind does not calm down until the end of the following night. The weather calms down in the evening while awaiting a new active Atlantic disturbance arriving in the second part of the night from Wednesday to Thursday. Minimum temperatures are falling and between 4 and 10 degrees in the South-West and Mediterranean coast. It is only between 0 and 3 degrees in the rest of the country. The maximums vary from 8 to 13 degrees on the Atlantic coast and the South-West, 13 to 16 degrees on the Mediterranean rim and 3 to 8 degrees on the rest of the territory.
Recommendations
Faced with these weather conditions, Météo France urges caution, particularly in the mountains and on coastal roads. It is advisable to postpone outings at altitude and avoid non-essential travel during periods of strong winds.
What is a strong wind?
A wind is generally considered violent and therefore dangerous when its speed reaches 80 km/h in average wind and 100 km/h in gusts inland. But this threshold varies depending on the region, for example it is higher for the coastal regions or the south-east region.
Strong winds have several origins:
The storms : In France, the diameter of storms is less than 1000 km. Storms coming from the Atlantic move quickly, up to 100 km/h. At one point, their duration does not exceed a few hours. At sea, we call a storm an atmospheric depression which generates an average wind greater than 90 km/h (force 10 Beaufort). On land, we speak of a storm when the depression generates gusts greater than 90 km/h.
Thunderstorms : They are the source of strong and brief winds (a few minutes) over a restricted area (a few square kilometers). Cumulonimbus clouds, characteristic storm clouds, driven by powerful vertical movements, create gusts of unpredictable direction.
In the mountains : The passage of wind over the summits can create violent gusts downwind, below.
Waterspouts and tornadoes : These whirlwind phenomena develop under a cumulonimbus and extend to land. These are fairly brief and very localized phenomena. Their lifespan does not exceed one hour, but several phenomena can occur in succession. We only talk about tornadoes on land.
The dangers
The damage varies depending on the nature of the wind generating phenomenon. Thunderstorm gusts cause damage of limited extent, waterspouts and tornadoes over a narrow and long band and storms or cyclones over a wider area.
Damage caused by strong winds:
– damaged roofs and chimneys
– trees uprooted
– vehicles shifted on the roads
– power and telephone outages
– Road traffic may also be disrupted, particularly on the secondary network in forest areas.
The wave-submersion phenomenon
Marine submersion can cause severe and rapid flooding of coastlines, ports and river mouths. They are linked to an extreme rise in sea level due to the combination of several phenomena:
Tide intensity : Sea level mainly due to astronomical phenomena and geographical configuration. The higher the coefficient, the higher the sea level at high tide.
The passage of a storm : Producing a rise in sea level through three main processes:
- strong swell or waves which contribute to increasing the water height;
- the wind which exerts friction on the surface of the water, which generates a modification of currents and sea level (accumulation of water as it approaches the coastline);
- the decrease in atmospheric pressure. The weight of the air then decreases on the sea surface and, mechanically, the sea level rises. A decrease in atmospheric pressure of one hectopascal (hPa) is approximately equivalent to a rise of one centimeter in the water level.
The breaking of waves results in a movement of water masses propagating on the foreshore (area covered and uncovered by the tide). Piers, dykes and other coastal infrastructure can then be breached, weakened or damaged.
The dangers
Strong waves and marine submersions are destructive phenomena, especially when they occur simultaneously. They can affect the entire mainland coastline, including in the Mediterranean where the tide is of low amplitude. Submersions mainly affect low-lying areas close to the coast. Floods due to marine submersion can, however, invade the coastline several kilometers inland and reach a water height of several meters. Communication routes, homes, and business areas are likely to be flooded and damaged in a few hours, or even less. Waves can damage coastal infrastructure (sea walls, jetties, etc.) and transport objects or materials (notably pebbles) which then become projectiles capable of injuring people, damaging property or obstructing traffic along the seaside. Objects not properly secured can be carried away. Boats, even moored to the pontoon in ports, can be lifted and taken onto dry land. Near estuaries, the flow of watercourses can also be slowed down or even stopped, which then generates overflows.
The damage can be aggravated in the event of violent gusts of wind, heavy rain, dike ruptures.
Damage to people and property caused by waves and submersion depends on natural factors but also on the implementation of human activities (land occupation). They can be reduced thanks to protective measures (dykes, jetties, dunes) and prevention (restrictions on developments in exposed areas, information, preparation, etc.).
