Here it comes, Chicago: Frost advisories and freeze warnings were issued for parts of the area, with temperatures dropping into the 30s overnight Friday and below freezing early Saturday morning.
In a briefing, the National Weather Service said the “coldest night of the season” was expected, with temperatures near or just below freezing across interior portions of northern Illinois, especially in counties to the north and west.
The frost advisory will go into effect at 5 a.m. Saturday for Lake, LaSalle and Kendall counties in Illinois, and Kenosha County in Wisconsin.
“Temperatures as low as 32 degrees will result in frost formation,” the NWS said. “Sensitive outdoor plants may be killed if left uncovered.”
In McHenry, DeKalb and Kane counties, a freeze warning will also take effect at 5 a.m. Saturday, with subfreezing temperatures expected.
Forecast models showed temperatures as low as 32 degrees in Hampshire, Huntley, Harvard and Waukegan. In Crystal Lake, temperatures Saturday morning will be 33 degrees. In Woodstock they’ll be even colder, with readings in the upper 20s, Sack said.
“Frost and freeze conditions could kill crops, other sensitive vegetation and possible damage unprotected outdoor plumbing,” the NWS said.
Both the warning and the advisory will expire at 8 a.m.
The warnings and advisories come a day after the NWS issued its 2026-27 winter outlook, with El Niño expected to significantly impact the region.
Weekend forecast
Friday started out dry, Sack said, though scattered sprinkles were possible through the morning and afternoon. In northwest Indiana, lake-effect rain showers will develop and last through the evening and into early Saturday morning.
Temperatures Friday will be in the mid-50s, Sack said, though wind gusts of 30 mph or higher will make conditions feel colder.
After a chilly morning, temperatures will remain in the 50s Saturday and rise into the 60s Sunday, with mostly sunny skies both days.
Frost advisory vs. freeze warning: What’s the difference?
With frost, temperatures drop to around 32 degrees, Sack said. Water freezes on the plants, potentially damaging them.
When a freeze warning is issued, temperatures drop to the upper 20s.
“The water freezes inside the plants, and that’s going to kill most plants,” Sack said.
When does the Chicago area typically see its first freeze?
The normal first freeze date at O’Hare, with 32 degrees or lower, is Oct. 19, the NWS said.
The first freeze in 2025 came much later, on Nov. 9, when temperatures hit 26 degrees.
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