For the past 7 years, we travel from just north of Kelowna,B.C. around Dec. 26th each winter heading to southern California.
We drive directly south on 97, through Omak, east Wenatchee, to Quincey, Moses Lake, down 17 to 395 through Kennewick, to I84 to Pendleton, La Grande and onto Twin Falls.
Travelled other routes, however this is by far the best route with good road conditions.
The major pass is just after Pendleton to La Grande, many times has snow, however DOT does a great job on maintaining the freeway.
Chains need to be carried and your truck tires should have the snowflake. One winter we got to Pendleton for the night and it snowed during the night. Next morning we could not go further unless we had chains for both truck and trailer....do not. We have snowflake truck tires and carry chains for the trailer, but could not go, so stayed an extra night. Usually what we have is enough and never had to put on my trailer chains. Have gone through a vehicle check for tires and chain possession and no problem. Units not properly setup get a steep fine.
Anyway, Oregon DOT has a good website with highway webcams.
A suggestion, once in Twin Falls, I would strongly consider heading due south on 93 to Ely, then Las Vegas and down to really fine warm weather.......just my two cents. Twin Falls is not warm in December, as I am sure you are aware.