HEAVY RAIN AND SNOW ALERT
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From Thursday 9 December at 12:00 to Saturday 11 December at 9:00.
The weather remains permanently disturbed in the southwest of France with several rainy bursts which follow one another until Saturday morning.
Sustained rains occur in the plains and abundant snowfall over the western Pyrenees with a large fluctuation in the rain / snow limit, leading to a high risk of avalanches in the mountains.
At the same time, an active disturbance gives snow temporarily in the plains or at low altitude (300-400 m) from the northeast to the Rhône-Alpes region at the end of the next night and until Friday 12 noon before the warm weather begins. raise the rain / snow limit.
Tonight and tonight: the rains continue in the southwest, in particular south of the Garonne and against the Pyrenees. The rain-snow limit was still located around 900 m altitude at 6 p.m., before the rapid but temporary ascent of the night to come. On this subject, let us insist on the avalanche risk become maximum (5/5) on the massifs of the west Pyrenees, and of 4/5 elsewhere. The rivers descending from the Pyrenees had started a significant increase on Thursday, in particular the Nive. A slow decline was observed on the Adour and the Gave de Pau and d’Oloron this Thursday, but the new rains of the next few hours should now reverse the trend with a further rise in levels expected in the coming hours. Overflows were also reported this Thursday evening in all the departments concerned by our alerts and the roads of the secondary network were now sometimes cut in the Pyrénées-Atlantiques. In this department, the hourly accumulations ranged from 1 to 8 mm, the strongest being in the Basque country. In 24 hours, it had fallen, for example, 45 mm in Bustince and 44 mm in Biarritz, where we had already exceeded 120 mm since December 1. In the mountains, the snow depths are very important, between 120 and 160 cm at an altitude of 1,500 meters.
This Wednesday, the accumulations of rain were significant over 24 hours with 43 mm in Pau, 46 mm in Orthez, 47 mm in Belus (Landes), 51 mm in Biarritz and up to 66 mm in Saint-Jean-de-Luz. This corresponds to 2 weeks of rain. The Adour was in flood but still far from the reference flood levels. The Gaves de Pau and Oloron were in slight decline after a peak in the morning flood.
Rain and snow parameter in the southwest
Tonight : the new oceanic rainy disturbance is spreading throughout the southwest of France, with new accumulations of 20 to 40 mm over the day. The rain-snow limit temporarily rises to around 2000 m over the western Pyrenees, 1,500 m over the Hautes-Pyrénées, and 1,300 m towards the east of the chain where the accumulations are lower.
Tonight : the heavy rains lasted (average intensity: 5 to 8 mm / h) all night long all along the Pyrenees and the rain / snow limit fell again to around 1300 meters at the end of the night. Note that the wind switches to the north-west sector and blows strongly with gusts around 90 km / h on the coast, which constitutes a brake on the river flow towards the sea.
This Friday morning: the rains gradually weaken and become more intermittent, but persist throughout the chain with a constantly falling rain / snow limit to reach around 1000 meters at the end of the morning.
Friday afternoon : a new, more active rainy period is expected in the second part of the afternoon with intensities again exceeding 10 to 15 mm / h temporarily. The rain-snow limit remains close to 1000 to 1200 m with thicknesses added to the imposing layer already present.
Friday evening and following night: A peak of rain intensity is expected during the evening with locally intensities above 20 mm / h. The rain / snow limit drops to around 800 meters on the eastern part of the Pyrenees. The rains gradually decrease in intensity the following night until Saturday morning.
Snow setting in the east
Tonight : the disturbance gains the west of the Grand-Est and Bourgogne-Franche-Comté around 3 am, giving snow from 200 to 300 meters above sea level. Snowfalls are moderate and hold up to the ground in temperatures close to 0 ° C. More significant accumulations are expected in the Ardennes then towards Luxembourg and the north of Lorraine.
This Friday morning: It continues to snow moderately from 300 to 400 meters from the Ardennes to the Lorraine plateaus and the snow is also reaching the Alpine foothills. In the second part of the morning, the warmth which gradually gains from the west raises the rain / snow limit beyond 400 to 500 m.
On this episode of rain and snow, we wait:
– cumulative rainfall that can reach 80 to 120 mm throughout the rainy episode and locally 150 mm, in particular in the Pyrénées-Atlantiques, in addition to the rains which have been falling on these same regions over the past two weeks. Thus, in this first decade of December, we will reach almost the monthly average of an entire month of December in Biarritz (average: 150 mm) and Pau (average: 98 mm).
– additional snow depths in the mountains of the order of 80 cm to 1.40 m over the western Pyrenees at 2000 m altitude, in addition to snow heights already close to 1.50 m at 2400 m. This will result in record thicknesses for this time of year. The avalanche risk, which fell due to the warmth of Tuesday, should then start to rise again in the coming days with the accumulation of new snow cover on unstable sub-layers.
– snow depths between 1-4 cm in the plains in the northeast, 5 to 10 cm from 400 meters on the Lorraine plateaus and in the Ardennes. In the northern Alps, the snow holds from 400 meters with possible accumulations of 5 to 10 cm as well.
List of departments concerned