Northern Illinoisans who spend a lot of time outside in the next day or so might be susceptible to frostbite if they don’t bundle up.

The National Weather Service issued a Cold Weather Advisory early Saturday for much of northern Illinois, including McHenry, Lake, Ogle, Lee, DeKalb, Kane, DuPage, La Salle, Grundy, Kankakee, northern, central and southern Cook County and northern, southern and eastern Will County.

The weather service said the advisory will run from 8 p.m. Saturday to noon Sunday.

Wind chills of 20 to 25 below zero are expected, and people should use caution when traveling outside. Those going outside should wear appropriate clothes, hats and gloves, according to the NWS.

“The dangerously cold wind chills as low as 25 below zero could cause frostbite on exposed skin in as little as 30 minutes,” according to the NWS.

The bitter cold follows snowfall in the region Saturday, although areas near and south of Interstate 80 have the greatest chance of hazardous travel because of the snow.

A cold weather advisory is issued when wind chills fall to minus 20 or below. A cold weather watch or warning can be issued if the wind chills fall to minus 30 or colder, but a watch or warning is not expected this weekend, NWS Meteorologist David King said.

Wind chills were below zero in many parts of northern Illinois on Saturday, but in Joliet, the wind chill was 2 degrees about 10:30 a.m. Saturday, King said.

Wind chills are expected to stay below zero until early Monday morning, King said.

Actual temperatures are expected to fall below zero overnight Saturday into Sunday, King said.

King said people should limit their time outdoors and bundle up if they go outside. He also said people should check on neighbors, relatives and senior citizens they know.

But the cold isn’t expected to last too long, and King said the weather should be much milder in the middle of the week next week.