Don't know if you've already made the trip or not? My hunch tells me you have. But just in case... the reality is that there is no way to get into Portland without encountering a mountain pass of some sort. Multiple passes actually, and you will encounter snow traveling through at least on of them. I-5 will take you through the Sisikou, (spelling?), range at the California, Oregon border. It's a crap shoot whether the conditions are good to treacherous, and the on occasion you will find it closed altogether. You will encounter numerous medium elevation passes during your first two hours after crossing into Oregon before finding relaxing easy driving once you enter the Willamette Valley and into Portland. On the other hand if its utilizing the fun handling of your car then you may find the last two hours into Portland as boring as the long California stretch of I-5.
You could probably find a route that could take you through central Oregon. Less mountainous drive yet beautiful scenery of the Cascade Range as you drive the mostly single lane high desert highway. Having just come from there I wouldn't advise it. I just spent many hours delicately washing the red pumice that was caked into my paint job. It is used as a traction aide in that region, as it is abundant and cheap, and not a nuisance only to those who could care less about preserving their paint vehicles paint job.
If time is not a factor, beautiful scenery a plus, utilizing BMW'S excellent handling qualities while, on occasion, pushing your own driving abilities Highway 101 can be a drivers delight. Very minimal elevation changes, majestic Redwood trees, magnificent ocean views ranging from sea level to cliffs high above the Pacific Ocean. Virtually no chance of snowy encounters or the damaging residue left behind by over zealous road crews always willing to litter the roads with some sort of gravel type substance, at the faintest snowfall, leaving it behind only relying on the steady flow of traffic to toss it about ricocheting off each other until eventually finding its way to the shoulders of the highways and out of harms way.
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