Can't tell from the picture, need a close up but stuff like that is typically Poa Annua.

Here are a couple more photos. I can Google, but if that's it, is there something I can use to kill it off and, more importantly, keep it from coming back?

Bummer, but that helps. Looks like I have a project in the fall. Thank you.

Yeah, looks like poa triv. Both love that PNW weather. Triv is extremely hard to eradicate--in my opinion, much more so than poa annua. I long for the day when Poacure is available and affordable as it has shown good results against both poa varieties.

I've got a few of those. Mine is triv. Lime green anywhere from 6-18" across.

Assuming you have cool season grass, the only thing that would work is Velocity...but it's $600 for a 1 lb bottle. Given that the triv is already a large portion of your lawn, even if you do go the Velocity route, you'd end up with lots of bare patches and having to overseed in the fall. Might as well just kill off the entire lawn and renovate in the fall.

Velocity is rarely used up north. It can damage cool seaaon grasses, especially Kentucky bluegrass.

Actually that's not true. According to Velocity's label, it can take out poa triv (roughstalk bluegrass) in creeping bentgrass, perennial rye, and tall fescue; all of which are cool season grasses. If the OP has Kentucky Bluegrass, then he could try Certainty. Although KBG is no longer listed on the Certainty label, many people have reported success using it in KBG.

Up North, kbg is king (with exceptions). You can't use it. Ive tried Certainty... That's a joke. You may as well use roundup which most people up north recommend.