October weather is about to give Chicago the cold shoulder.

       Temperatures, which have been mild and unseasonably warm, will turn shivery Thursday night and become nippy again Friday and Saturday nights, too.

       However, an updated outlook for this winter predicts slightly warmer than normal temperatures and a higher probability that rain could stick around into early next year thanks to the return of the La Ni？a climate pattern.

       1st freeze

       The area is still waiting to record its first freeze of the fall season.

       According to Brett Borchardt, meteorologist for the National Weather Service’s Chicago office, we’re running just a bit behind normal for this milestone at O’Hare International Airport, the city’s official recording site.

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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.

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       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.

       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.”

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       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 inch 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 through February, according to an update from the Climate Prediction Center on Thursday.

       (NOAA Climate.gov, using NWS CPC data)

       (NOAA Climate.gov based on NWS CPC data)

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