First frost of season possible Saturday morning

Temperatures could dip below freezing early Saturday morning in areas outside Cook County, bringing a brief interruption to the Chicago area’s fall weather.

The National Weather Service warned that the coldest temperatures of the season, so far, are expected this weekend. A frost advisory was issued for Lake County, north of Chicago, and Kendall County southwest of the city. Freeze advisories were issued for areas farther west in McHenry and Kane counties.

While Chicago is unlikely to see frost, temperatures could drop lower in areas farther from the city, according to Kevin Doom, a National Weather Service meteorologist.

Temperatures are expected to fall into the 30s overnight Friday and dip to around freezing early Saturday morning before warming back up by early afternoon.

“It’s going to be pretty brisk tonight,” Doom said.

The freezing temperatures will be short-lived. Temperatures will continue to rise through the weekend and back into the 70s by early next week, Doom said.

This year’s first freeze is arriving slightly earlier than usual. The average date of the first freeze in Cook County is Oct. 23, according to Doom.

Despite the brief cold snap, a potentially “historically strong” El Niño could bring mild winter temperatures to Chicago.

An El Niño is a weather event that occurs when abnormally warm water temperatures in the Pacific lead to changes in weather patterns globally, specifically warmer temperatures across northern North America.

For the Chicago area, that means a 50% to 60% chance of above-normal temperatures this winter. However, the weather service said even in a mild winter to expect “notable” stretches of cold weather and snowstorms.