From personal experience when I was younger: Spending a month traveling on European trains with a backpack will teach you a whole lot about what you actually need to pack, and what you will want to just leave behind and purchase again locally if you really turn out to need a given item.

For me, the starting point that makes a lot of it way easier is to focus on layering. Anyone who lives in the Bay Area will instantly understand it: you don't build outfits for different weather, you build outfits for all weather that just depend on how many layers you've got on. For example, no cold-weather coats: instead you pack a feather-light and very foldable rain shell, layer it over thin sweaters for cold weather (multiple, if it's cold enough), and wear it directly over a light outfit for warm rainy weather.

I tried layering when I traveled somewhere cold, where the high was just above freezing. I felt foolish whenever I went indoors and had to peel off three or four layers in order to feel comfortable.

The next time I went, I brought a proper winter jacket. When I was indoors, I took off my jacket. It was simpler and more comfortable, both outdoors and indoors. You cannot replicate a winter jacket with a rain shell and a bunch of layers.

The Bay Area has a lot of microclimates but none of them are really cold nor hot or humid.

The heaviness of your layering will depend directly upon the environment you will be in. The above is adequate for most temperate climates.

If you plan on being at higher elevations during local fall/winter/spring, your base layer will likely be light (and moisture wicking, please do this), and your first layer will likely need to start heavier than a temperate climate base layer.

Having spent a considerable amount of time in the North Central US during those seasons, the layering strategy is appropriate, but the heavier outer layers are non-negotiable, especially as the average temperature drops below 15F.

In late 1970s, I spent two months hitching and riding trains around Europe, just an external frame backpack and a sleeping bag. A few years ago, I spent 2.5 weeks in U.K. and Ireland, with an 18-liter backpack and a waist bag (a.k.a. fanny pack ), which made getting around a breeze especially at airports and on trains. It even allowed me to carry a notebook computer. As mentioned in TFA, merino wool and no cotton garments helps. The rain was a minor inconvenience, but I did have lodging where I could get things rinsed and dried out in my room. I did buy an umbrella but should have splurged on better quality, even if I would end up abandoning it.

Best packing advice I ever received: take half as many clothes, and twice as much money, as you think you'll need.

There are certainly circumstances where you need really warm clothes. And hiking (and requirements for dress clothing) complicate thingx as well. But if I'm visiting cities and towns in not surefrigid weather, I can generally handle carry-on with some washing out in sinks and so forth with the right low maintenance clothing.