Invasions of chilly air from the north become increasingly common during the second half of September and October. Up north, the air is growing colder by the day, mainly because all effective sunlight has been lost. Up around the Arctic Circle, the sun is barely above the horizon this time of year, and outgoing heat energy greatly exceeds what is being gained.

Friday, expect a touch of autumn across the northern Great Plains and the Prairies of Canada with highs ranging from the 60s in South Dakota to the 40s in Saskatchewan. Chilly air will reach the Midwest and Great Lakes Friday night and Saturday. Some of this cooler weather will be felt across the Northeast Sunday and Monday.

Say, are those snowflakes on the graphic above? Indeed they are! Summer's grip is slipping fast.

Story by AccuWeather.com Senior Meteorologist John Kocet.