There is a possibility of snow no matter how you go. We have gone through Tri-cities through Pendleton, Boise etc and it was a bad trip the snow over immigrant pass was really scary. Last year we went eat on I-90 and then south from Butte we encountered snow that way also.
My recommendation is to check the long range forecasts pick a time and go on the I-90 route. Only travel when you are comfortable. There are lots of rest stops along the way if it gets bad stop. Don't push it.
Worst case is you stop at a motel and "hole up" until the snow passes.
The nice thing is once you get South of Salt Lake City the weather gets noticeably warmer and you will start to relax.
jesse
