US President Joe Biden was due to visit Louisiana this Friday, the first state to be hit by Hurricane Ida, where he should once again call for union in the face of climate change. This visit comes as, further north, the region of New York is still healing its wounds, bruised by sudden storm flooding, which killed at least 44 people. According to Frédéric Nathan, forecasting engineer at Météo France, the phenomena that caused this disaster could occur in France.
How can we explain that Cyclone Ida retained such strength from New Orleans to New York, nearly 2,000 km further north?
FRÉDÉRIC NATHAN. Cyclones lose power when they hit a shore because their engine, which causes them to gain power, is warm maritime air. Ida did not escape this rule and also lost a little energy but, classified in category 4 (Editor’s note: on a scale from 1 to 5 depending on their power) when he arrived on the coast of Louisiana, it continued to produce extremely heavy rains. His route took him to the northeastern United States, where unfortunately the highly urbanized area of New York is located.
What happened to make the level of precipitation so high in the American megalopolis?
Another phenomenon occurred: the depression created by Ida encountered in this area a mass of cold air at altitude called a thalweg. The association with the always hot air at low altitude causes a very stormy situation, like that of the cold drops which we have talked about a lot in France this summer. The result was stormy rains, or convective, with very heavy precipitation since the historical record was broken in New York: 80 mm of water in just one hour.
Could the floods have been avoided or anticipated to limit the damage?
Unfortunately no. There is nothing to be done because with such a volume of precipitation, flooding is inevitable. When this type of stormy rains fall in rural areas, soils and rivers can partially absorb them. On the other hand, in ultra-urbanized areas like New York, the overflow is inevitable because the saturation of basements and sanitation networks is too rapid and impossible to contain.
VIDEO. “It’s unheard of”: Hurricane Ida kills at least 44 in New York and its region
The very heavy rains which fell on New York on August 21, by beating the previous record, did they amplify the phenomenon and prevent the good flow of this new precipitation?
No, because drainage systems in urban areas play their game in the hours and days that follow after being submerged, unlike what can happen in rural areas where already waterlogged soils can have difficulty. to absorb a new supply of precipitation.
Could the city of Paris be struck by a similar situation?
Sure ! We are absolutely not immune to a huge storm, in Paris as elsewhere. In France, we already know such levels of rainfall during the Cévennes episodes. When it rains on the high plateau of the Ardèche, the consequences are much less dramatic than when a storm hits a city. This has already happened in the 1980s with disasters in Nîmes or Montpellier. Without being able to say with precision what the consequences of a thunderstorm of an intensity equal to that which hit New York would be, it is a safe bet that the Paris metro would be flooded … as was that of the American megalopolis. .
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