What is a Pneumonia Front?

October 4, 2023

You may have heard the term 'pneumonia front' thrown around on social media recently. It's a phrase that's actually been used by meteorologists for decades, and it refers to a type of back-door cold front that sweeps down Lake Michigan in late spring. It can cause temperatures to plummet in a matter of minutes, dropping 20 degrees or more. These types of fronts are exclusive to Wisconsin, Illinois, Indiana and Michigan, because they only occur when the air over the chilly lake waters is cooler than the air a few miles inland.

On Tuesday, a pneumonia front raced down Lake Michigan, bringing a quick end to a summerlike day in Milwaukee and Chicago. After Milwaukee topped out at 81 degrees early in the afternoon, the temperature plunged to 62 degrees within an hour. AccuWeather RealFeel was in the upper 40s, and it felt more like early June than late spring.

These fast temperature drops are due to the fact that wind flowing over a large body of water moves much faster than it does over land, because it has less resistance. In addition, the chilly air produced over the icy lake water pushes against warmer air from the west and southwardly races into communities along Lake Michigan. It's this rapid change in weather conditions that earned the pneumonia front its local name coined by the National Weather Service in Milwaukee in the 1960s.

This pneumonia front is expected to arrive in southeast Wisconsin this evening. Temperatures are forecast to drop from 80 degrees in Milwaukee and Sheboygan to 60s and 50s inland in a short amount of time. The temperature will rebound by Thursday, but a light jacket won't be out of place for the remainder of the week.


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