An early season snowfall occurred across portions of northern Illinois on Tuesday. This was made possible due to a more winter like pattern than one that is common for October. Temperatures will continue below average for the rest of the week.
A large ridge of upper high pressure has built over the west. This has caused the jet stream to buckle into the eastern United States from Canada, causing cold air to pour in. A small but vigorous low pressure developed along this jet stream in Canada and dropped into the Midwest. The system then quickly moved through the Ohio Valley. This is often called an Alberta Clipper, named after where it originates in Canada. Systems like these are more typical from December through March.
Light rain associated with this system broke out across Iowa and Illinois. Temperatures were generally in the upper 30's. As the rain fell into the drier surface air, it evaporated. This took heat out of the air and led to further cooling. As this occurred, the rain changed to snow. The heaviest bands of snow fell across the southern counties. The National Weather Service in Chicago shows that portions of LaSalle county had 1-2 inches of snow. O'Hare Airport and Aurora had traces of snow. This is about one week earlier than the average first snow.