Coup de Galerne
I have just had a weather warning about a 'coup de galerne' that is due to hit the south west coasts of France today (5th May 2020), which will then roll northwards and eastwards across France, bringing violent thunderstorms, fierce gusts of winds, hail, heavy rain etc, etc (we're DOOMED I tell ye, DOOMED!).
OK, I'll be unplugging the router tonight! But what is a coup de galerne? I'll let Fanny explain: -
Word of the day 'époustouflant' - breathtaking. On prends sa serviette et on décamp! Grab your towel and leg it!
This is a rather drier definition I found online: -
During the passage of a cold front coming from the Bay of Biscay, the westerly winds are reinforced by being channeled by the Cantabrian Mountains, along the Spanish coasts, towards the Basque Country. The galerne therefore begins with a rapid descent of atmospheric pressure and temperatures - up to 15° C in 20 minutes - with very strong, very cold winds of up to 120 kph accompanied by intense rain and hail. Changes in temperature and humidity are caused by air from the Atlantic, which replaces that from the land, and atmospheric convection is made possible by the frontal uplift caused by the advance of the colder air.
This video gives a very clear idea of how fast and how violent these changes can be. All of this took place in just 20 mins - compressed in the video to 4 mins.
A couple more images from the weather forecast with the consequences for us here in Normandy tonight and tomorrow morning. I shall be making sure everything is under cover and protected from any hail!
It's a phenomenon known as an arcus cloud, which are relatively rare. Aa arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus thunderstorm. Roll clouds and shelf clouds are the two main types of arcus. Arcus clouds most frequently form along the leading edge or "gust fronts" of thunderstorm outflow; some of the most dramatic arcus formations mark the gust fronts of *derecho-producing convective systems. Roll clouds also may arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.
*A derecho is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of severe thunderstorms known as a mesoscale convective system. They can cause hurricane-force winds, tornadoes, heavy rains, and flash floods.