People should protect their homes now against storm damage as much of France is at higher risk of violent weather and flash flooding this autumn due to drought and warmer sea temperatures.
Experts warn that the particularly dry soil, which struggles to absorb rain, creates a serious flooding risk.
‘The first rains, especially if they are violent, will run off the soil’
“The first rains, especially if they are violent, will run off the soil,” said Françoise Vimeux, climatologist at the state research body Institut de recherche pour le développement.
This is a common problem in urban areas since roads and pavements cannot absorb the water but the drought means rural areas are now also at risk.
“The run-off will end up in rivers, which may become swollen, and we could witness flash floods following violent storms.”
Many areas already suffered floods last month and five people died when a sudden and violent storm, with winds of up to 220km/h, hit Corsica.
Épisodes méditerranéens or épisodes cévenols
There is a particular heightened risk in the south with storms known as épisodes méditerranéens or épisodes cévenols and a campaign has been launched in 15 departments to advise residents what to do in the event of resulting floods.
Ecology Minister Christophe Béchu said: “Nine million residents could fall victim to these cévenol storms, which can reach up to 200 litres of rain per square metre.”
People should now check and clear guttering which might be blocked by leaves or debris and, if leaving a second home, turn off the power supply.
Instructions during a storm
Instructions during a storm include keeping to higher floors, turning off the gas, electricity and heating, and leaving children at school where they are safe if it is too dangerous to go out.
A spokesperson for France’s firefighters says that when floods occur, people often put themselves in danger by trying to save possessions – especially by going down to check cellars. Instead, they should move any important items out of cellars now and on to a higher floor.
Dr Vimeux said épisodes méditerranéens are common in the autumn as water that evaporates from the sea and is carried inland meets a mass of cold air, resulting in heavy rain.
A warm sea means water is more easily evaporated, and when the atmosphere is warm, it is able to hold more water vapour, which can lead to more intense rain than usual.
“In the western Mediterranean, we have seen temperatures of 2C to 6.5C above average,” she said.
CNRS researcher Florian Pantillon said: “What has happened over past weeks, with several heatwaves, is quickly forgotten by the atmosphere but the inertia is much higher in the ocean. If the sea is warm, it will remain warm for a while.”
This is not certain to continue, Dr Vimeux said.
“The surface stays warm, as there is little wind and so it has not been mixing with deeper waters. If we have strong winds across a large section of the sea, it could reduce the temperature.” Dr Pantillon said that, in extreme cases, warm sea temperatures can contribute to the formation of ‘medicanes’ (‘Mediterranean hurricanes’), similar to tropical cyclones but on a smaller scale.
These have struck Greece and Sicily in recent years.
He added: “The probability is maybe higher than usual, but still very low, because medicanes are rare. On average we see less than one a year.”
Expected heavy rains will not solve drought instantly
Water shortages were another problem in France this summer, and the expected heavy rain will not provide an instant solution to this due to the probability of the first rains running off the dry ground surface.
Dr Vimeux said: “It is clear that the return to normal will take time, and will depend on the type and amount of rain that we see.”
While conditions will vary year on year, she said the trend is towards warmer summers with decreasing amounts of rain, meaning this summer could provide a glimpse of those to come.