I checked with my Pawnee National Grasslands authority and we agreed this was 2016.  I have been told that some of these wildflower phenomenons happen in five year cycles.  One year primrose, one year wild yellow flowers, one year wild purple.  I  have held the yellow coreopsis to this schedule.  This 2016 display of white blossoms made the entire grassland look like fields of snow.  Memorable!