Austin, TX to Portland, Oregon any suggestions?

Stay South until you get to I5. North on I5 you really have only the Siskiyou's to worry about and in most cased the last two weeks of Dec are still a little early to be real concerned about a major snow storm. Not to say it can't happen it is just a little early for a real heavy snow storm and in most cases if you have to if you lay over for a day or two in Redding you can make it across. Carry chains, if you get stopped and don't have them it is a heavy fine.