Do I need a heavy-duty down parka in Edinburgh in mid-December?

You should be fine with what you already have. Just make certain your base layer keeps you toasty and the waterproof shell keeps you dry.

What works in Seattle should work in Edinburgh, especially with the underneath layers you describe. In Edinburgh there may be more wind chill than you're used to, simply because the city is hilly and especially the castle end of the Royal Mile is quite exposed. It will likely rain, and you could get "lashing rain" (wind driven) so a down parka without a waterproof shell would not be the ideal outerwear.

(I actually wasn't aware they made down parkas in waterproof -- would that be Gore-Tex or something similar? My experience with these breathable water-repellent fabrics is that they may be water repellent for awhile but are not really waterproof, hence somewhat vulnerable to getting soaked in a downpour.)

Thank you, Claudia and epltd.

epltd: The waterproof down parka I was considering is the North Face Arctic Parka. https://www.thenorthface.com/shop/womens-arctic-parka-ii-nf0a35bq#hero=0 It's not Gore-Tex; it's something NF calls Dry-Vent. But I don't actually have the jacket so I don't know whether it would hold up to the rain, over time.

the important thing is to have something that is windproof. it is easy enough to put an extra layer or two on under any jacket but if it is windy then the temperatures will feel a great deal colder.
I actually bought a heavy duty parka several years ago and gave it away after one winter as it was far too warm for me.

You can always wear layers, but a good pair of thick soled waterproof shoes/ hiking boots and thick socks, hat, scarf and gloves are also essential.

I have no idea why you think you are going to get a “bit of snow on one day” - the forecasters can’t get it right in so much detail for 5 days ahead, much less a month! The weather is all over the place at the moment, so historic averages mean nothing at present. Last week we had 17 degrees and it’s going to be 6 this week, so they predict. The long term average is 10 degrees.

I have a Musto down parka that I use on the colder days, when going hiking or in cities at nighttime, but it gets less use in cities during the day as it’s too warm.

Thank you all for taking the time to comment. Very helpful!

I was surprised at that stylish North Face parka. Looks like a miserable thing to be forced to carry around. If you’re in Seattle, you’ve already got everything you need. If you’re a cyclist, you have more than enough. Maybe you need a merino wool skin layer for some extra insulation. Horizontal rain is possible in any exposed area so you either take your rain pants, hooded shell, buff, and gloves (and find a convenient place to layer up) or you slog it out, knowing you’ll have a nice cup of tea or coffee and a warm place to get dry within a few hours.

Happy travels. Pack light!

What works in Seattle will work in Scotland. Our climates our similar. However, there is no way that a weather forecast for Scotland can be accurate that far out! Our weather is extremely changeable. The temperature range looks about average but who knows whether it will snow?!

Thank you all!

What to pack/wear for cool season travel should be a FS FAQ packing tips topic but winter travel is not high on the RS touring schedule.

The general answer to you question is layering - base layer (next to skin), insulating layer (could be down) and a outer wind/rain protection layer. REI's expert advice is a good starting point to answering you question:
https://www.rei.com/learn/expert-advice/layering-basics.html

Note that when you refer to a "waterproof down parka", "waterproof" can be either a parka with a built in waterproof shell, or hydroscopic down. The primary disadantage of normal down insulation is not working when wet. The down can get wet from external sources rain or falling into a body of water, or interan sources (perspiration). Down garmets filled with hydroscopic down that has been treated for water repellency tend to be expensive.

If your parka is built with a waterproof outer layer, it will be less "breathable" than a jacket built with a open weave and more likely to trap moisture. A waterproof (but breathable) outer jacket constructed with vents is the better way to go.

Now all that said, walking about Edinburgh is not even close to peak bagging Glen Nevis and an even an Arthur's Seat walk in Edinburgh during winter is not likely to be life threatening. What works in Seattle should be good enough and if not, a good opportunity to buy a Scottish sweater.

When I was in Edinburgh over New Year I was wearing jeans, a polo shirt and a lightweight waterproof jacket but I don't really feel the cold, besides the walking warms you up and being in museums, pubs, restaurants etc all with the heating on full meant that I actually enjoyed coming out into the cool air.

I just got back from Scotland and it rained pretty much the whole time with temps in the low-mid 40s. The key is a waterproof shell (which sounds like your current set-up). I even left my only fleece jacket on the plane, but I just doubled up on the clothes and I was never cold. I'm sure it would have been a different story without a waterproof shell, though.

Just got back yesterday from 10 days in Edinburgh.
Don't take down, it'll just get wet.
The things that were the most useful were a thermal long sleeved top under everything; a pair of fleece -lined tights under my outer jeans/ pants/trousers; a warm scarf wrapped right round my neck; a toque/hat; and fleece gloves.
I didn't use my umbrella in the rain, but put my hood up instead.
You need a good waterproof jacket, and good walking shoes.
I also loved my merino wool socks that I brought.
The temps. were not any colder for the time of year , about the same as here in Vancouver; but Scotland has a creeping dampness in winter, and Edinburgh is often windy.
A lot of museums have lockers to put your coat and bag in as you go about the museum; so that's always handy.
And a lot have great cafes with home made soups and baking too!
Have a great trip!

Thanks to all of you who responded to my question. I'm back from Edinburgh and, as many of you advised, what works in Seattle worked just fine in Edinburgh. My Thermoball parka, with a light waterproof shell over it, and suitable layers underneath (especially merino wool base layer), plus gloves and hat, worked really well. I think if I had gotten the heavier-duty down parka, that I would've been overheated. Thanks again, all!

Please tell us how you liked Edinburgh, and what you did there?!