"It will be winter soon!"
Yeah, this is actually really interesting. This was the hardest thing for me to wrap my mind around when I was a first year student, and my teacher said "了 marks completion, but it isn't necessarily past tense."
But yeah, the way I chose to think about it is that it is marking imminent completion, as in, this will be completed in the future. The 快 lets me know it won't be long.
It can be used with other units of time, for example:
一年后我会到了。 I will arrive in one year. 今年春天什么时候会到了？ When will spring arrive this year. 我一会儿就到了！ I’ll be there in a second!