Just a thought...If the weather is too treacherous when you leave, you could always cut over to HWY 101 in Eugene. We drove this route a couple of times, though not in the winter. The road is more windy and make take you a couple more hours of travel time, but I don't recall any real mountain passes. You do have to cross the Golden Gate in San Francisco and the traffic can be pretty bad, but we have always enjoyed this route.
Just please please please be careful, and have MAPS, lots and lots of good maps, if you're going to go west like that. Thinking of that family that got lost last year trying to cut over to the west like that.
I've done the December drive down to the Bay Area many times in years past, and it's NOT fun! I don't know if Oregon still allows triple-trailers, but if they do, that's heinous to be passed on a curvy stretch of road at night by one, two, three trailers... Hopefully they don't allow that anymore, but I don't know.
Even without the triple trailers, the weather can be very rotten, and in the mountains, no matter what time, it can be tricky. I never encountered the need for chains, and thank goodness b/c I never traveled with them, but I *could have* needed them at any time.
Even Seattle to Portland is iffy at night...it's a pretty boring stretch of road once you get past Olympia, and the speed increases to 70 (which of course means people go 80 with no problem), and did I menton it's boring?
I like driving, and I will drive that route...I just won't do it in one shot (to the bay area...I'd never drive to So CA in one shot, LOL) anymore!
All that said, we're taking Southwest for our trip rather than driving. With my particular three year old, it's just too long in the car (for him and with him, ha ha) for a drive like that.
