Invasions of chilly air from the north become increasingly common during thesecond half of September and October. Up north, the air is growing colder bythe day, mainly because all effective sunlight has been lost. Up around theArctic Circle, the sun is barely above the horizon this time of year, andoutgoing heat energy greatly exceeds what is being gained.Friday, expect a touch of autumn across the northern Great Plains and thePrairies of Canada with highs ranging from the 60s in South Dakota to the 40sin Saskatchewan. Chilly air will reach the Midwest and Great Lakes Friday nightand Saturday. Some of this cooler weather will be felt across the NortheastSunday and Monday.

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

Story by Senior Meteorologist John Kocet.