Hi Janie:
We headed down on Nov 3 and before Great Falls ran into heavy snow so we ended up staying in Great Falls for a couple days as the passes at Helena and Butte were not good. It just wasn't worth the risk.
After the 2 days, the highways were cleared and travel was good. You could see where there had been many vehicles in the ditches before.
We just came back; Dec 3; from Yuma with just the truck and around midnight; this past Sunday we ran into a wicked snow storm and very high winds at Monida Pass (Montana and Idaho border). Vehicles in the ditches, lucky I got to follow a highways plow truck up and over the pass so we slowly made it through. This storm just came up like you "flipped" a switch.
It's not best to travel through that area in the late night. You never know what will hit and when.
We also managed to get through Salt Lake City before they got hit with a wicked snow storm. It's bad enough going through their in good conditions, let alone a snow storm.
So as others have posted, you have really keep watch of weather reports and if necessary slow way down or better yet just park.
Good luck and be carefull.