Unfortunately we can't ID this to species, as we don't see enough distinguishing characteristics. Given when it appeared and the fact that winter didn't kill it, it's either a cool-season annual or a perennial. From what we can see, it doesn't appear to be barnyardgrass or orchardgrass. Letting some go to seed would help narrow-down the candidates, but we realize that's not the ideal solution. Grassy weed ID is challenging and relies on fine details to separate similar species.

If you'd like us to take another look, we'd need to see features such as vernation (how new leaves emerge), the ligule (at the leaf bases), the collar (same area), and whether the leaf surface has any hairs at any point along its length. Your close-up photos on the plain background are great, but we can't quite make out these particular structures. Peeling back entire leaves from the main stem will allow you to see these features more easily. Examples of what these structural traits look like are shown and explained in these grass ID keys (though your particular weed isn't necessarily on these key listings because these are largely desirable turfgrass types, though these traits are shared among all grasses):

• Turfgrass Identification (PDF)
• Turfgrass Key

The good news is that regardless of specific ID, many grassy weeds won't tolerate the mowing height of typical fescue maintenance, so will eventually die-out on their own. For those that do, you'll have to spot-treat any clumps with systemic herbicide, which for that extensive of an area, may not be practical compared to simply renovating those patches and replacing them with sod for a fresh start. Systemic herbicides for grassy weeds in lawn also tend to kill any fescue they contact as well, but product specifics can vary and you'd need to see what its label instructs.

Miri

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Hello Robert,

Thank you for the additional pictures. We had one of our weed experts look at them and he thinks it's a type of cereal rye. He agrees that they should die back fairly quickly on their own after the lawn is mowed a few times.

Once the fescue resumes vigorous growth as our weather warms (and we get more rain) again, it should start to fill-in any areas where this weedy grass has colonized the lawn. You can also overseed any thin areas this spring if you wish, though it may not be necessary. A spring fertilization (follow the fertilizer law regarding quantity, use a phosphorus-free formulation, and our recommended schedule) could also help speed-along turf establishment as the rye declines, though this too may not be necessary for the lawn to "repair" itself on its own.

Miri