The big thing is to allow time for delays due to weather. That said the comment to head south on I-5 is a good one IF you are in western WA. If not let me know as there are some alternatives. My previous home was in MT and we went south most winters till a few years ago. And we have travelled to OLY/Portland to visit family for the holidays and then headed south on I-5. The Siskiyou's are always an issue in the winter but are doable if you allow time and can either miss the storms OR duck through when chains are not required. We stayed near Medford a couple of times and did just that, ducked through when chains were not required.
Ounce south of the Siskiyou's, the only risk of snow will be the mountains in southern CA, again if snow happens it will soon be gone. Make your way from Bakersfield across to San Bernardino then sought to I-10 and across the US on it. Long trip, don't push too hard and stop every 5 or 6 days for housekeeping, laundry and to just rest up. Expect ot average about 400+ miles a day so that the trip is not tooo burdensome. I our case we plan 350+/- miles per day. Sometimes a few more and seldom less than 325 when we are are on a forced trip to meet a schedule. Don't get me wrong, we have done over 600 miles in a day but that was due to a family emergency. It is no fun and extremely draining on all in the vehicle.
Hope this help and if I can answer any questions, you can contact me via email.