That can be a tough one, Alvah.
The first step is to make sure youve done a good job of layering, as that can be a very effective strategy. For a trip such as you describe, Id start with a very light, thin layer like Patagonias Capilene One ($39 for long-sleeve). Over that, a midweight wool shirt like Icebreakers Chakra Zip ($80). Then a midweight fleece like REIs Alpine Meadows ($80). And finally, a windproof shell. Try Outdoor Research Aspire ($200).
About half that for your legstwo light layers, and a shell. That should be fine.
Now, when youre moving around you can mix and match that set. Light layer with fleece over it. Light layer with shell. Light layer alone... But I will say, when you are stopped for a snack or lunch, I tend to think you need one more thing: a good insulated piece. As my friend Larry says, "Instant warmth."
So take a look at adding a light down piece to your set. A down sweater might be adequate, something such as MontBells U.L Down Inner Jacket ($150), which offers an amazing amount of insulation in a six-ounce package. Yes, six ounces. Or, consider a more classic down jacket that you just throw over everything when you come to a stop. Here, take a look at Mountain Hardwears Sub Zero SL Hooded Jacket ($275).
Or, if you have the budget for it, get a Feathered Friends Hooded Volant Jacket ($339 with Epic Nextec shell; $399 with waterproof/breathable eVent shell). This is a serious winter piecean amazing amount of warmth in 22 ounces. And an excellent safety backup should you get holed up in a snow cave during a blizzard.