A horse will generally grow as much coat as it needs. She will fluff her coat to trap heat coming off her body. A blanket will flatten her coat. This throws her thermostat out of whack.
They typically do not even get the winters I get in southern Middle Tennessee; I have gotten enough snow that I had to plow our 500' drive
If the weather folks are correct and we do end up with single digit temps, I would also worry in that climate. Up North my horses used to stand outside and make icicles out of themselves and they were fine. They were also a lot younger and without the health issues they now have.
If I were the OP and don't have a barn to shut the horse in at night, I would want a blanket on standby if the nights do turn raw and bitter, even if it never comes out of the bag. No point in running the redline on the shivers and colic. The wide weather swing causes enough problems on its own for some horses