Tom Bennett wrote:
"Gianna" wrote With one seemingly imminent I was wondering where the term
comes from.

According to Chambers, it is of American origin.

N. American. I always understood that it is a warm, dry settled spell on
The Prairies in autumn, that often comes before winter sets in. And there,
I think, any similarity between us and continental USA ends.

True Tom, but it is a term much over-used by the press here, and should
really only be used once we are in October or November.

Despite the climatological definition of autumn, early September should
be still capable of late summer warmth that is anything but
exceptional; we even had 34C+ in September 1911 in Coventry!

Steve J
BWS
Coventry