Meanwhile, in the Midwest, with September snowstorms causing temperatures to plunge by more than 60 degrees in some regions. Up to 17 feet of snow fell in mountainous areas of Wyoming on Labor Day - which marks the unofficial end of summer. Snow also fell in nearby Yellowstone National Park, with a bison seen walking through sleet and icy winds in one video shared on social media. 'Labor Day in Yellowstone. Surely this bison is a metaphor for something in 2020,' Twitter user Chris Frink wrote beneath the short clip. Snow in Denver last night and right now.