Thank you for your ID and for your helpful comments. Like you, I was wondering about seeing an ant with wings in September. In 2009 I photographed one on August 15th:

I've never seen them swarm here as early as June though. Yesterday the temperature reached 25⁰C. It's very unusual for the temperature to go that high here in Newfoundland in September. Do you think the high temperature could have something to do with a September swarm? The location of the photograph is less than 100 meters above sea level.

On a personal note, you mention the Jura Mountains. My Great grandfather, Augustin Erbland, was born around the area of Sondersdorf in southern Alsace. I guess that's not too far from where you live? I enjoy genealogy almost as much as 'bugs'. :)

is indeed a possible explanation, if not the only one.

Southern Alsace is not "too far" from where I live at a North-American scale (around 120 miles, since I live in Nyon, between Geneva and Lausanne).
But I must confess I've never been in Alsace (except the border town of Saint-Louis, near Basel where I went several times).
I spend most of my holidays in the Southern or Western half or France, and I igored even the existence of Sondersdorf. I'll look for it at Google Maps!