Snow mold. Treatment is just a raking area to break up the grass blade and let it air out.  Normally recovers after 3-4 mows.  Leaving it untouched could cause dead spots that will need to be seeded.

Please call right away if you see Armyworm, they can destroy your whole yard.

“Fall armyworms have arrived in bigger numbers in Nebraska”

- omaha.com

“Fall armyworm moths have migrated north into Nebraska, and the damage they do to your lawn can be devastating. In one of the biggest outbreaks across the United States in 40 years, armyworms are feasting on large swaths of turf grass, leaving brown and dead lawns behind them.”