My question is, why? Aside from possible snow in a few places, you can do it, but you won't have time to see much.

its possible but you may run into snow on I-5. Hwy 101 is doable but slower. When compared with flying or even taking the train economically it doesn't make sense but can you do it? sure.

In December, why go north? If it's snow you're after, then head for a mountain and ski. California has good skiing. Most of the attractions are closed and most of the views are buried in clouds. Three days is certainly no problem (we used to get from Vancouver to Disneyland in three days), but not sure what you are hoping to accomplish on this trip. You get mountain passes in northern California, and you are probably not experienced driving in the snow (I've been to WA, so I know you don't get much snow there ;>), so this could be hazardous if you get caught in a storm. Give some serious thought to the risks of travelling this time of year and, if you decide to do it, make sure you are prepared for getting caught in a snow storm.

I assume from your post that you have an extra 3 days to go somewhere and you thought about Seattle. Seattle is wonderful but too far for a 3 day turn-around trip. It is a little over 800 miles one way. It might take 13-15 hours of driving. You would have to use the inland interstate freeways (5) to do it which precludes enjoying the coastal scenery. Seattle is on the Puget Sound and NOT on the Pacific coastline. Once you got there you would have to decide if you wanted to visit Seattle or take another road trip to the ocean. It might be better for you to stay in the SF Bay Area and make use of the Bay Area Rapid Transit System and visit other Bay Area Cities: Oakland, Orinda, Walnut Creek... or take the Cal Trans commuter trains south to San Jose or north to Sacramento and visit the State Capitol. You might also try a ferry to Sausalito for a day.

Whilst others mentioned the possibility of snow on the mountain passes, I will tell you about the RAIN!

If you are thinking about seeing the Oregon coast in December, be aware that it will be raining. A lot. We usually get about 12" of rain just in the month of December - that's about 300 millimeters.

If you are looking to just travel somewhere different with three extra days (?), try the No Calif coast and see the redwoods. It will be raining there too, but not nearly as much.

Yeah, I didn't get your post - do you have 3 days TOTAL to turn around and get back to SF or is it three days you have to get up to Seattle.

It will take you 12 - 13 hours to get from SF (leave AM) to Seattle if you haul it up 1-5. Its an all day affair.

again, give us more details. 3 days one way or two way?

Good suggestion half-Brit. A day trip north to Muir Woods or a day trip south along coast Hwy 1 to Monterey might fill the bill for Didi. If you wanted to continue South you could get to Hearst Castle area (San Luis Obispo/Moro Bay) for an overnight trip.