Attached are two additional photos. As you can see our lawn has also been overrun with nutsedge

I'd suggest Conyza canadensis, aka Horseweed.

Thanks Floral_uk! I may try to pull it before it gets much taller but realize I may need to resort to an herbicide (as I will with the nutsedge). At least now I have a name.

Close mowing and thickening up the lawn should deal with it. The grass is very long and rather sparse in the picture and the weeds are getting a chance.