A heavy downpour fell on Vannes on Tuesday around 8 p.m. Which municipalities were affected and how many millimeters fell?
Station readings show enormous disparities over a micro-area. In that of Séné, there was only 6 mm between 8 p.m. and 9 p.m. But that of Plumergat displays 20 mm. And by satellite observations, we can see that it is the northwest of Vannes which was particularly affected, in particular the Ménimur district, the south of Plescop and the village of Béléan in Ploeren. There, there was what we call a flood, a very heavy accumulation of rain in a short time, which we can estimate at 70 to 90 mm. It’s exceptional… The norm in Vannes is 48.4 mm of precipitation over the entire month of June. In some places, almost two months of rain fell in one hour.
There are mobile storms that move around, but that wasn’t the case here. A southerly wind faced a northeasterly wind, which led to the formation of a cumulonimbus very loaded with water. And it remained stationary in this area.
Thursday, we will have to carefully monitor the situation in Morbihan, from Lorient to Redon via the Gulf and Vannes
Météo France had only placed Morbihan on yellow alert from this Wednesday, 6 a.m. Was there a failure?
They were a little late. But as of Tuesday morning, Météo Bretagne had issued a yellow level bad weather risk map covering South Brittany.
As soon as there is yellow alert, you have to be very careful, especially for thunderstorms, because these are very localized phenomena. The situation can quickly degenerate and become dangerous!
Is there a risk of the same phenomenon happening this Wednesday and the next few days in Brittany? Can we predict where exactly?
This Wednesday, it is rather the Loire-Atlantique which will be affected by the storms. In Brittany, there may be stormy precipitation, but it will not be of comparable intensity to what Vannes experienced on Tuesday evening.
On Thursday, we will have to carefully monitor the situation in Morbihan, from Lorient to Redon via the Gulf and Vannes. The models show very sustained precipitation which could fall locally over a very short period of time. This could happen from 4-5 p.m. until 10-11 p.m.
Friday the weather will be calmer. A new, more traditional disturbance will arrive in the evening: the Music Festival should unfortunately be rained out, especially in the west of Brittany. Saturday, we will still have a trailing sky. On Sunday, things will start to get better. Then, Monday and Tuesday, it will be summer weather. For the future, it is uncertain.
