Tuesday could bring a double whammy to Chicago — fall’s first freezing temperature and first snowfall.
If both are observed, then it would be just the 10th time in local weather history the two milestones were achieved on the same calendar day, says Jake Petr, a meteorologist in the National Weather Service’s Chicago office.
Temperatures will be in the high 20s to low 30s Tuesday morning with wind gusts up to 25 mph.
Snow showers will start after 11 a.m., with an accumulation of more than a trace but less than an inch.
Then expect wind chills in the 20s Tuesday afternoon — quite the difference from highs in the 50-degree range Chicago has experienced on Halloween the past three years.
Chicago experiences higher temperatures longer than outlying suburbs due to the urban 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.
Here’s a look back at when our area typically experiences its first freeze and first snowfall of fall and what to expect the rest of this season.
The earliest first freeze on record occurred Sept. 22, 1995, at O’Hare International Airport, the city’s official recording site. The latest was Nov. 24, 1931.
The first freeze didn’t arrive until Nov. 11 last year.
“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,” Brett Borchardt, meteorologist for the National Weather Service’s Chicago office, told the Tribune.
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.”
Fall’s first measurable snowfall — you know, the stuff that sticks — is expected to arrive Tuesday. If a tenth of an inch or more accumulates, then that counts as “measurable.”
If there is less than a tenth of an inch observed at O’Hare then that counts as a “trace” amount — even if it doesn’t stick. Whether a trace amount of snow is a “first snow” is up for debate.
Chicago received its first measurable snowfall on Nov. 15 last year.
[ Chicago’s 10 largest snowfalls since 1886 — and how the Tribune covered them ]
After a “triple dip”, La Niña is gone. Conditions are favoring warmer-than-average temperatures due to the arrival of the El Niño weather phenomenon, according to the National Weather Service’s Climate Prediction Center.
But first up — bitter cold.
Sources: National Weather Service; Tribune reporting and archives
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