Chicago experienced its first freeze of the fall season this morning.
From 6:35 a.m. through 7:42 a.m., 32 degrees was recorded at O’Hare International Airport, the city’s official recording site.
Lew Carlaw, meteorologist for the National Weather Service’s Chicago office, says the early morning timing of the freeze is appropriate.
“Generally speaking, lows tend to occur right around sunrise,” he said. “It takes time for heat generated during the day to burn off (overnight).”
What’s normal
According to Brett Borchardt, another meteorologist for the National Weather Service’s Chicago office, we’re running just a bit behind normal for this milestone.
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“The first freeze, which is when the temperature drops at or below 32 degrees, typically occurs between Oct. 11 and Oct. 12 across the Chicago suburbs and Oct. 21 to Oct. 30 in the city and along the lakeshore,” he said.
The earliest first freeze on record occurred Sept. 22, 1995, at O’Hare. The latest was Nov. 24, 1931.
Frost can develop on clear nights when the air temperature is in the mid-30s, but can be scattered. That’s why, WGN-TV chief meteorologist Tom Skilling says, the weather service “does not keep statistics regarding frost but instead uses the season’s first temperature of 32 (degrees) or lower to define the end of the growing season.”
The forecast
Carlaw says the forecast for tonight is even colder — by about 3-4 degrees. Downtown Chicago could flirt with a freeze tonight, he says.
Chicago experiences higher temperatures longer than outlying suburbs due to the heat-island effect. Its location next to Lake Michigan’s warm waters explains why the city and nearby suburbs freeze later in the year than their farther-out counterparts.
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This week’s below-average temperatures are the first time since mid-July that Chicago has experienced five days in a row below average, according to the weather service. Warmer temperatures are expected to return next week.
1st snow
“The first trace or more of snow typically occurs on Oct. 30, but has occurred as early as Sept. 25 (1942) and as late as Dec. 5 (1999),” Borchardt said. “The first measurable snow (0.1 inches or more) typically occurs around Nov. 17.”
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Last year, Chicago received its first measurable snowfall of the season on Nov. 24.
What to expect this fall
The latest outlook from the National Weather Service’s Climate Prediction Center shows a transition has begun to the La Ni？a climate pattern for the second consecutive year, or a “double dip,” as meteorologists call it.
As a result, the Chicago area could experience warmer weather and more rain — or not.
“La Ni？a falls tend to be a bit drier and warmer across the Midwest, but there is A LOT of variability between La Ni？a years,” Trent Ford, Illinois state climatologist, wrote in an email to the Tribune.
What to expect this winter
Warmer temperatures but wetter-than-average conditions in the Great Lakes region are expected from December 2021 through February 2022, according to an update from the Climate Prediction Center.
This graphic from NOAA's Climate Prediction Center shows the U.S. Temperature Outlook for the upcoming winter. (NOAA/Climate Prediction Center)
This graphic from NOAA's Climate Prediction Center shows the U.S. Precipitation Outlook for the upcoming winter. (NOAA/Climate Prediction Center)
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