Collection location: Santa Rosa, Sonoma Co., California, USA [Click for map]
Found on a lawn in downtown Santa Rosa. These seem more fibrillose than other examples on this site of L. leucothites.
I mentioned the observation to Else, and she wrote back “that is typical Leucoagaricus barssii – it tends to be early – in other words it fruits in drier periods than the other species.”
I think of that species being grayer, but it made sense after she mentioned it.