Hope you have a great trip. If you should happen to need something extra, there are a few shops in those cities, I’ve heard.

Good luck! I'm actually loaning one of my Icebreaker merinos to a friend as we speak (for an upcoming UK trip) since, like you, she doesn't want to buy a whole new wardrobe for inclement weather whilst on holiday.

How exciting! I’ve done this kind of travel a lot and never take more than a carry on. I also don’t pile up on layers for the airport/train because I tend to get hot on the move, so some heavier pieces have to be able to fit in the bag. I’m always happier under packing. As others have said, I would start with footwear and outerwear. White sneakers and ankle boots will take you everywhere, and you’ve already put those in your finds. I tend to avoid longline cardigans on trips because they are bulky to pack, and stick with pull over sweaters, cotton or wool depending on the season, tees with short or long sleeves or no sleeves to layer underneath also depending on temps. Don’t forget about pjs and even slippers, because sometimes the coldest part of a travel day is the lodging, which Europeans do not keep as warm as Americans. Good socks and a stylish knit hat also go far. For this, I’d be tempted to take my 3-in-1 Vosque coat from Patagonia. If you don’t have something similar, think about investing. Have a great trip!

Since you run warm, I’d dress like I was coming up to San Francisco during winter…

Jenn- I am like you, I tolerate cool/cold weather well and actually like feeling on the edge of a slight chill. I hate feeling hot. In Spain @ the temps you will likely encounter I was comfortable in a light weight sweater or heavier long sleeve tee with a light weight raincoat in the 50's. I didn't take a cardigan but that would have worked also. Jeans and mid weight ponte pants were perfect. I didn't take any yoga type pants but they would worked also. I felt a little warm in the heavier weight ponte in the 60's + but they were OK in the mid to low 50's. I did need the light weight puffer a few nights when it dropped into the low 40's. I would have felt a little too cool in a dress or a skirt with bare legs so would have wanted some tights or legging underneath.
Hope this helps.
BTW J Jill wherever pants worked well for me. They dried fairly quickly. https://www.jjill.com/product/.....?color=780

Definitely a packable puffer and a packable raincoat- like the rain cape below. Seconding Athleta- they have things that pass as regular, non-gear clothes that look travel friendly.

Suntiger, that rain cape is awesome!

Lynn, I used to have those same J.Jill pants! My ponte pants from Brass are quite similar so I'll add those to the list. I appreciate the insights from someone who also runs warm.

notsaf, I'm ALWAYS hot on planes and I suspect masking isn't going to help matters. Good advice on the lodging! As far as coats, I'm pretty set on bringing my trench. It's lined and hooded and my puffer fits beneath if need be. I never get to wear it in California!

rachylou, I've never been to San Francisco in the winter, just May and July.

Sounds like an amazing trip! I've been dreaming of a multi-week trip on another continent. It's the one part of pre-pandemic life that we haven't quite gotten back to yet. I hope you're able to catch the tulips in the Netherlands!

My experience of northern Europe in early spring is that it's not all that different from spring in New England: occasionally very pleasant, more often than not chilly, windy, and damp. Coastal areas and higher elevations will feel colder. For temps in the 40s-50s F, with the occasional sunny day in the 60s, I'd want hat/gloves/scarf and thermal baselayers (I prefer thin silk over synthetic or merino; I find it does a much better job avoiding that overheated feeling when you're transitioning between outdoor and indoor spaces). Granted, I do run cold, and I do like to be prepared to spend all day outdoors, whether in an urban or rural setting.

Definitely bring a fully waterproof/windproof coat with room for layers underneath. Definitely bring your most comfortable walking/shoes boots with decent water resistance (I hesitate to call anything short of a rubber boot truly waterproof, but you want something less clunky and more breathable than that). I would probably stick to pants and avoid skirts/dresses, but only because I am so prone to ripping and snagging tights that I find dresses aren't practical for travel outside of midsummer temps when I can wear them with bare legs.

Have you thought about your luggage situation? Do you have a good day bag for walking around? In transitional weather, I like having a tote/daypack/large crossbody with me so I have someplace to stash waterproof layers on days when the weather is iffy, and to stow my accessories and layers if it warms up in the afternoon.

A note mostly for my own reference.  It's 48° and 80% humidity in LA today and I was comfortable walking outside in jeans, a ls cotton tee, and my puffer.  If it were windy, I'd also want a hat and scarf.

Sounds good. Also looks like warmer clothing than I was picturing somehow