A 'Pneumonia Front' Will Hit Chicago Sunday, Creating A Possible Contrast Between T-Shirts And Jackets

Sometimes the Great Lakes can act like a refrigerator. This weekend will offer an example when a so-called pneumonia front sweeps into Chicago.

What's Happening

Don't let the name fool you. This cold front won't make you sick.

The term "pneumonia front" is specific to southeast Wisconsin and northeast Illinois and refers to a dramatic drop in temperatures in the Chicago and Milwaukee areas helped by Lake Michigan. In this case, it refers to a cold front like we'll see Sunday pushing across Lake Michigan, causing northeast winds to flow off the chilly Lake Michigan waters into northeast Illinois, northwest Indiana and eastern Wisconsin.

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The Result Is Miles Separating Jackets From T-Shirts

At some point Sunday afternoon, temperatures could range from the 50s and possibly upper 40s near Chicago's lakeshore to the 60s and 70s in far southern and western suburbs like Joliet and Aurora. Eventually, temperatures will crash to colder levels in those outer suburbs as the pneumonia front continues to push inland.

In the past, this has forced people near the lakeshore to quickly put on a warm jacket after potentially starting the day without one, or even in a t-shirt, sometimes over the course of a single hour.

Examples Of The Impact

Milwaukee Mitchell International Airport saw its temperature drop from 75 degrees to 45 in one hour during an April 10, 2017, pneumonia front event.

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The setup can also allow a deck of low clouds and/or fog to invade southeast Wisconsin and northeast Illinois, as the satellite loop from the June 1, 2018, event shows below.

Why It Happens

These pneumonia fronts are most common in spring, when Lake Michigan's waters are chilly. A buoy in the waters off southeast Wisconsin showed temperatures were in the mid-40s on Friday.

The cold water helps chill the air above it, which is then pulled into the lakeshore behind fronts approaching from the northeast. When springlike temperatures are in place, the resulting plunge can be shocking over the span of just an hour.

Where The Term Came From

The term was first coined by the National Weather Service-Milwaukee in the 1960s. Essentially, this is a much stronger version of the more typical lake-breeze front that helps air-condition the lakeshore in the heart of summer.

Chris Dolce has been a senior digital meteorologist with weather.com for nearly 15 years after beginning his career with The Weather Channel in the early 2000s.