What is a 'pneumonia front' and why is it unique to southeast Wisconsin?

MILWAUKEE -- You may have heard the term "pneumonia front" thrown around a lot in the last week by meteorologists in Milwaukee, but what does it even mean?

In short, it's a regionally termed phenomenon you won't find in meteorology textbooks coined in the 1960s by Milwaukee's very own National Weather Service office at the time. It's a cold front with enhanced cold air thanks to chilly Lake Michigan temperatures at the surface and typically occurs in early spring.

As it moves through eastern Wisconsin through northeastern Illinois, it causes a drop in temperatures of at least 16˚F in one hour, but even this number is debatable.

Pneumonia Cold Front Definition

What this kind of boundary is considered by meteorologists in textbooks is a "back door" cold front. It's a cold front that has an eastern component due to where the low is located. In many cases, when we see a back door cold front, there is a low off to our NE in Michigan or Ontario.

This is exactly how our regionally coined pneumonia front moves. It first starts in Green Bay and uses the cold pool of air over Lake Michigan to bow through the edge of Wisconsin, causing a rapid temperature drop to inland cities.

Cold front progression of a Pneumonia Front

If we analyze our most recent pneumonia front event on Sunday, May 3 -- it was a beautiful afternoon. Temps climbed into the 70s and was one of the nicest days so far in 2020. The front entered Oostburg, and that air was quickly 22˚F colder than Cedarburg -- just one county away.

Temps Sunday afternoon, May 3rd

As the boundary continued to clear, the area temps dropped everywhere, but especially closer to the lake! In many cases, we dropped well above 16˚F in an hour and it truly was the regionally termed pneumonia front.

Sunday, May 3rd evening temperatures

What's with the scary name? Some theories are that it's linked with one of the symptoms of pneumonia, which is chills, and how the chilly temperature drop is pneumonia-like if you're outside. Other explanations include cold air moving through the lung shaped Lake Michigan as if an infection is rapidly moving through, just like how pneumonia spreads in the human body.

We'll leave the medical terminology to the doctors, but one thing is for sure -- Milwaukee and areas surrounding Lake Michigan do get some strange weather!