I assume this is a German colloquialism. It would be nice if the app noted such.
I would not call it a colloquialism. It is good German and you can very well write or say in in formal circumstances.
“Im Anzug sein” means “be about to arrive” and it is usually used with threatening things like a storm or (jokingly) your mother in law.
“Anzug” here has nothing to do with the business suit, it is connected to “heranziehen” i.e. “approaching”.
Thank you for the explanation. I was, in fact, confusing “Anzug” in this sentence with the word for business suit.
That said, I change my comment: it would be nice if the app noted the use of a homonym.