Dan, you may consider canceling the trip...according to what I've seen on the tube, to all intents an purposes, Portland has been shut down. That includes the airport.
Click around the road cams here:
http://www.tripcheck.com/Pages/RCMap.asp?mainNav=RoadConditions&curRegion=1
I'd suggest doing so in the Portland area where there are lights. To give you a clue,,I-84 east of troutdale has been closed.
What the cams don't show is the sheet ice under the snow. I-5 is essentially a skating rink through the Portland area.
Amtrack has cancelled it's runs from Portland to Seattle...
If you insist on coming anyway, buy the rental car insurance,,,because the odds are high you'll bounce around a bit...unless you can rent a jeep with all 4 wheels chained.