In Milwaukee, we cherish warm days almost as if they were our children. So it’s the ultimate meteorological betrayal when, on a blessed spring or summer afternoon, it suddenly feels like the gales of November came early. You might not know the term, but you’ve experienced this uniquely Great Lakes phenomenon: a pneumonia front.
One moment, it’s 76 degrees, all skirts and shirtsleeves. Then comes the unmistakably unseasonable wind, blowing north to south. Within an hour, the temperature has plummeted to 60, or even lower. “A cooler-by-the-lake day,” explains CBS 58 meteorologist Sam Kuffel, “where the cold’s going to punch you in the face.”
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Pneumonia front isn’t in the National Weather Service glossary, but forecasters use the term for a temperature drop of 16 degrees or more in one hour.
The front occurs in spring or early summer, typically only one or two times a year, and can be felt as far west as Jefferson County. It’s triggered when there is relatively cold Lake Michigan water and relatively warm air. “The change in temperature becomes more drastic than if Lake Michigan wasn’t there,” says Tim Halbach, a meteorologist in the Milwaukee/Sullivan office of the National Weather Service.
That office is credited with coining the term for these “back-door” cold fronts in the 1960s, but the service doesn’t officially record them. Maybe you remember a couple in Milwaukee: The temp dropped from 82 to 66 between about 6 and 7 p.m. on June 17, 2022; and 75 to 45 between roughly 1 and 2 p.m. on April 10, 2017.
Our relatively mild winter means Lake Michigan should warm up more quickly this spring, but keep the extra hoodie ready for another pneumonia front. “I wouldn’t rule out that it would happen,” Halbach says.
