This Monday, the 40 ° C were crossed in the middle of the afternoon, in Bégaar in the Landes. “At 12 p.m., we already had 38.8°C in Orthez (Pyrénées-Atlantiques), and 38°C in Belin-Béliet (Gironde)” points out Francis Morisset, forecaster and departmental climatologist for Meteo France in Biarritz, “and 2.5 degrees more in Biarritz than on Sunday at the same time. »
Dozens of temperature records for the month of September, dating back several years, were falling on Monday afternoon throughout the Southwest quarter. “The latest records are 40.5°C on September 7, 2016 in Cambo-les-Bains (Pyrénées-Atlantiques), points out the forecaster. On September 7, 1966 we had 40.2°C in Arjuzanx and 40°C in Parentis (Landes). We can also note 39.8 ° C on September 14, 1962 in Saint-André-de-Seignanx and 37 ° C in Bordeaux on September 17, 1967. ” The average maximum temperatures in September in New Aquitaine, gravitate rather between 23 ° C and 25°C.
A peak of heat which occurs after three heat waves
The forecaster, however, believes that “we should not go into heat wave orange vigilance, because the phenomenon will not last long enough. This is why we speak of a heat peak. But it comes after three waves of heat wave since June. We can also point out that 40°C has been exceeded… for the fourth month in a row.
This peak of heat crosses the Southwest “despite greater cloud cover than Sunday” also points out Francis Morisset. “This is due to instabilities coming up from the Bay of Biscay which will give rise to high clouds. We are currently affected by a depression, the former cyclone Danielle seen off the Azores a few days ago. This air, crossing the Pyrenees, undergoes a foehn effect, that is, the mass of air compresses, which warms it and dries it. We had the same situation, more or less, during the heat wave in June, with a depression located off Portugal which brought in a flow from the south, bringing air from North Africa. It is also possible that the air mass will become increasingly sandy, as in June. »
Watch out for high altitude thunderstorms
From Tuesday, however, temperatures should begin to drop, “even if they will still remain above 30°C. » « The problem is to know if we will have rain, because we are expecting thunderstorms at altitude. These are located at about 4,000 meters and its precipitation evaporates from 3,000 meters, with a risk of lightning. This situation can therefore create sparks, with a wind that could attract embers, so be careful of the risk of fire, especially in the Landes and Gironde where the ground is still very dry. »
A new heat peak “is likely to recur next week” warns the forecaster, “with however lower temperatures, but which will still remain above seasonal norms. »