The Hautes-Pyrénées, Gers and Pyrénées-Atlantiques are placed on orange alert by Météo France. Thunderstorms, heavy rain and floods with even hail are forecast for Saturday June 8 and Sunday June 9.
Météo France has placed the departments of Hautes-Pyrénées, Gers and Pyrénées-Atlantiques on orange rain, flood and thunderstorm alert this Saturday June 8 and Sunday June 9.
Current situation
The event has not started.
General situation : the region is affected by a warm and humid air mass. Due to a high altitude depression coming up from Spain, this air became unstable late Saturday afternoon to give rise to heavily rainy thunderstorms in the west of the region.
Predictability and uncertainties
During the evening of Saturday and the following night, a stormy deterioration sets in over the southwest of Midi-Pyrénées. These storms are associated with high electrical and rain intensities. Hail is also expected. Uncertainties remain regarding the location of these most virulent storms.
Departments on the fringes of those on orange alert may also be affected by locally strong phenomena.
Thunderstorms
Intense rain-storm episode requiring particular vigilance, in particular due to the very high accumulation of precipitation locally over the period.
Expected development
Strong storms will break out this evening, starting on the Pyrenees, and quickly spilling over into the foothills or the plains. Heavy rain is expected in a short time, with hail and electrical activity at times intense. These storms will gradually subside after midnight.
Rain Flood
Intense rain-storm episode requiring particular vigilance, in particular due to the very high accumulation of precipitation locally over the period.
Expected development
During the evening of Saturday and the following night, a stormy deterioration begins in the south of the region. These storms will be associated with high rainfall intensities, which can reach 20 to 40 mm/h.
Rainfall accumulations of 40 to 60 mm can be reached in a relatively short time, or even very locally 60 to 90 mm. Over the entire episode, the expected accumulations may reach or exceed 100 mm.
After the strong thunderstorms during the evening of Saturday, more continuous and therefore less intense rains are forecast until the morning of Sunday, which could therefore cause flooding problems in places.
Over the entire episode, the expected accumulations may reach or exceed 100 mm.
For the day of Saturday June 8
For the day of Sunday June 9
The rain-flood phenomenon
Intense rains bring a very large quantity of water over a short period of time (from one hour to a day).
This quantity can equal that usually received in one month (normal monthly) or in several months.
In the South of France, the accumulations observed can exceed 500 mm (1 mm = 1 liter/m2) in 24 hours. For the most violent phenomena, the accumulation exceeds 100 mm in one hour. Heavy precipitation can result from several weather phenomena:
violent and stationary thunderstorms
a succession of localized storms
a disturbance associated with widespread rain
Rain running off and concentrating in watercourses can cause flooding. Flooding can be due to a slow rise in water levels in lowland areas, the rapid formation of torrential floods or rain runoff.
The extent of the flood depends on three parameters: the water height, the speed of the current and the duration of the flood. These parameters are conditioned by precipitation, but also by the state of the watershed and the characteristics of the watercourse.
In Mediterranean regions, particularly during autumn, rises of warm and humid air can cause accumulations of several hundred liters of water/m2 in just a few hours. Stormy rains in the south-east of the Massif Central cause significant flooding in this region (for example: the catastrophic floods of Nîmes in September 2005 where a cumulative amount of nearly 430 mm was reached).
The entire territory of mainland France can be exposed to intense rain. However, they are more common in the Mediterranean regions and the Alps.
Intense rains can cause devastating flash floods, particularly in mountainous areas (Vaison-la-Romaine on September 22, 1992, Le Grand-Bornand on July 14, 1987).
On the contrary, it may be that it no longer rains but that the watercourse nevertheless leaves its bed; this is particularly the case for so-called “slow flood” rivers such as the Seine or for example the Somme in 2001.
Large-scale floods are usually the result of persistent, intense rainfall. The danger is amplified in winter, when there is little evaporation and the soils are saturated with water. The melting of the snowpack also contributes to raising river levels. Rainwater runs off into rivers, too quickly to flow away, and they overflow their banks. These events are more common in Mediterranean regions and the Alps.
Rain of moderate intensity, which lasts several days, can also cause flooding by a slow and gradual rise in water levels, this is the case for rivers such as the Seine.
Dangers
The risk of flooding is the conjunction of a phenomenon – the water of a river in flood which can then escape from its usual flow channel – and an exposure – human activity installed in the alluvial space (constructions, equipment and activities). The damage caused by flooding therefore depends on natural factors (relief, nature and state of water saturation of the soil) but also on the implementation of human activities (land occupation). They can be reduced thanks to protection measures (dykes) and prevention (flood retention zone, development of areas to be flooded, information, preparation, etc.).
The phenomenon can go unnoticed in a sparsely populated area.
Cities also experience high water runoff due to waterproofed surfaces. On the other hand, saturation of the rainwater drainage network can cause overflows and considerable damage (example: Nîmes, October 3, 1988).
Finally, the damage can be aggravated by other factors: violent gusts of wind, landslides, dike ruptures, hail, strong waves.
What is a thunderstorm?
A thunderstorm is an atmospheric phenomenon characterized by lightning and thunder. It is always linked to the presence of a cumulonimbus type cloud, also called a thunderstorm cloud, and is often accompanied by a set of violent phenomena: gusts of wind, intense rain, sometimes hail, waterspout and tornado.
The cumulonimbus is a cloud with a diameter of 5 to 10 km, very developed vertically, which can rise up to 16 km in altitude. At its summit, the cumulonimbus spreads out widely, which gives it its general anvil shape.
A storm can always be dangerous at a given point, due to the power of the phenomena it produces.
The storm is generally a short-lived phenomenon, lasting a few tens of minutes to a few hours. It can be isolated (storm near reliefs or caused by warming of the ground in summer) or organized in a line (called “squall line” by meteorologists). Under certain conditions, storms can regenerate, always in the same place, causing heavy precipitation for several hours, leading to catastrophic flooding.
Dangers
Lightning is the name given to a flash of lightning when it hits the ground. This intense electrical discharge can kill a man or an animal, burn a tree or cause fires.
The intense rains that accompany thunderstorms can cause devastating flash floods. A cumulonimbus 1 km wide by 10 km high contains 1 million liters of water.
Hail, precipitation made up of small pieces of ice, can devastate a vineyard or orchard in just a few minutes.
The wind under a cumulonimbus blows in violent gusts up to around 140 km/h and changes direction frequently. More rarely, a very devastating whirlwind, the tornado, is created under the base of the cloud.
