At 4:38 a.m., the temperature hit 35 degrees, tying a record set in 1895, said meteorologist Kevin Birk.
The normal low temperature for this time of year is 48 degrees, and the normal high is 69 degrees, said Meteorologist Matt Friedlein. The weather should warm up Monday, with highs hitting the mid to upper 60s across Chicago.
"The main reason is due to the anomalous cold air mass that shifted over the Great Lake region," Birk said.
The National Weather Service issued a Frost Advisory for the entire Chicago area from 1 a.m. to 8 a.m. Sunday.
ABC7 Meteorologist Larry Mowry said overnight lows dipped to the low-to-mid 30s away from downtown Chicago and Lake Michigan.
"We probably won't see much frost in the heart of the city thanks to the heat island effect," Mowry said. "The farther north you go, that's where the coldest temperatures will be."
Plants that are sensitive to frost should be covered or brought indoors.
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The Sun-Times Media Wire contributed to this report.