Weather alert: heavy rains in the Pyrenees and snow in the east – La Chaîne Météo

From Thursday 9 December at 12:00 to Saturday 11 December at 9:00.

Situation

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 in particular in the western Pyrenees with a large fluctuation of 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 plain or at low altitude (300-400 m) from the northeast to the east of the Rhône-Alpes region the next night until Friday 12 noon before the warmth begins. raise the rain / snow limit on Friday.

Observation

This Thursday
AT 12 : light to moderate rains persist in the west of the Pyrenees with intensities of 1 to 2 mm / h bringing the accumulations to 39.1 mm in Biarritz over the last 24 hours. The level of the rivers has stabilized for the moment see continues to fall in particular for the Gaves of Oloron and Pau, but the new rains this afternoon will reverse the trend by this evening.

At 9 o’clock : the rains extend from the Aquitaine coast to the Pyrénées-Atlantiques where they are often heavy. The rain / snow limit goes up little by little to reach around 1000 m due to the warmth which is imposed by the Atlantic. Over the last 24 hours, it has fallen on average 30 to 40 mm from the south of the Landes to the west of the Pyrénées-Atlantiques. The slow decline of the Adour or the Gave de Pau and Oloron continued overnight, but the new rains of the next few hours should now reverse the trend with a new flood expected in the afternoon and the evening.

Wednesday

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.

Evolution

Rain and snow parameter in the southwest

This afternoon : the rains coming from the Atlantic are reinforced on the Pyrénées-Atlantiques while progressing again on the south of the Landes and the Hautes-Pyrénées with intensities which can reach 10 to 15 mm / h temporarily, even up to 20 mm / h on the Basque country. At the same time, the rain-snow limit will rise between 1500 and 1700 m of altitude with an increase in snowfall above, and rain below. At that time, we will have to be wary of the reaction of the torrents descending from the Pyrénées-Atlantiques, where the contributions of a partial melt will combine with the rains. The rains will also extend further east of the chain, towards Ariège, but initially remaining moderate.

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 rises temporarily above 2000 m over the western Pyrenees, 1,700 m over the Hautes-Pyrénées, and 1,400 m towards the east of the chain where the accumulations will be less.

Tonight : the heavy rains persist (average intensity: 5 to 8 mm / h): all night long all along the Pyrenees chain and the rain / snow limit drops again to around 1300 meters at the end of the night. Note that the wind shifts to the north-west sector and blows strongly with gusts around 90 km / h.

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.

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

Next night : the disturbance gains the west of the Grand-Est and Burgundy-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.

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 warm weather which gradually gains from the west raises the rain / snow limit beyond 400 to 500 m.

On this episode of rain and snow, we expect:

– 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