Hard snow in October.
Next year, hard snow in September. The year after that, hard snow in August.
After a while, summer just kind of goes missing altogether. Snow that fell in April never really melts. It piles up a little before the summer in August. Snow plows can't keep up with that kind of demand and some roads don't get cleared before the end of spring and have to be "abandoned" as have many roads now in Canada.
Each year the snow gets a bit deeper and the first snow of winter gets a little larger in volume until it is getting kind of ridiculous and nobody has the leisure or means to clear it away and one day you're looking at this white blanket of snow and thinking somewhere under there lie the ruins of a 21st century technological society that doesn't show above the snowline anymore. Of course by this time there are not all that many people left to look at the snow and think this because most of them have died.