I think it is Chamerion angustifolium, sometimes called Fireweed. Pull it as soon as you can.
Not to be confused with Epilobium angustifolium, another, larger 'Fireweed'. It's easier controlled even though it spreads by seeds and underground runners.

I have both, plus a third, smaller one that I can't be sure of the Latin name. It looks like a smaller form of Chamerion angustifolium.

Chamerion angustifolium and Epilobium a. are the same plant.

bboy, can you explain?

All I know is, there is a large one (six feet plus)that grows in burned over areas in the Cascades, sometimes found in suburban gardens. It's the large one that can spread by underground runners and by seed.
Then there is a medium sized one that is commonly found in gardens here in the lowland Puget Sound basin. A annual weed of disturbed ground, found almost always near chickweed, but usually in a little bit drier ground. No runners.
There is also a smaller one, hardly different than the medium sized one, that likes wetter places in the garden. It has smaller leaves and doesn't get as tall. Again, no runners.
I'll let you sort out the Latin. I tried. There seems to be lot of overlap and miss identification.

There are many species of Epilobium, some of which have had name changes, like the fireweed. Some are native, some are introduced weeds. There's only one fireweed though, by whatever name. Watson's willowherb is a common garden weed, I think it's Epilobium ciliatum now, used to be Epilobium watsonii. Dry or damp, sun or shade, tall and rank or short and sparse. Seeds, no runners. The one in your garden is probably the Watson's although you may also have others like Epilobium minutum in dry spots. I'm sure bboy is much more knowledgeable about the various species than I am, I usually do well enough just going by 'Epilobium species'.

What you have in the photos on the Weeds forum sure look like Watson's willowherb gone to seed. They've done really well this year with the cool damp spring. But the photos do need to be larger to really see. I'm just guessing from the silhouette.

The Burke Herbarium web site has two of them in Chamerion and 15 others in Epilobium. The two Chamerion differ from the others in producing comparatively big, flat and conspicuous flowers.

A.L. Jacobson studied and identified all of the fireweeds known by him to be present in the area of coverage when preparing his wild plant book.

Here is a link that might be useful: Wild Plants of Greater Seattle