Pneumonia Front Invades Milwaukee, Chicago. Here’s What That Is.


Temperatures and winds during the passage of a pneumonia front along the Lake Michigan shoreline on June 1, 2018.

  • A cold front enhanced by Lake Michigan brought big temperature changes Friday morning.
  • Temperatures plunged almost 20 degrees in less than two hours in several spots.
  • Low clouds and brisk winds over 30 mph swept in behind the so-called pneumonia front.

A recent stretch of hot, humid weather was brought to a halt Friday morning as a so-called pneumonia front sent temperatures plunging along the Lake Michigan shoreline.

The cold push of air refrigerated by Lake Michigan surged southward along the Wisconsin shoreline early Friday.

In less than two hours, the temperature at Milwaukee’s Mitchell International Airport, less than three miles from shore, plummeted from a warm 72 degrees just before 4 a.m. to a more typically cool for early June 54 degrees just after 5:30 a.m.

Adding to the abrupt change, winds gusted from 30 to 40 mph along the lakeshore in Milwaukee, Racine and Kenosha Counties. So much for leaving the light jacket in the closet.

Behind the cold front, a solid deck of low clouds socked in the previously clear skyline of Milwaukee, as seen in a timelapse tweeted by WTMJ TV meteorologist Josh Wurster.

Because of this low cloud deck, the pneumonia front stood out well in visible satellite imagery as it surged into eastern Wisconsin and northern Illinois.

The front then plunged into Chicago, erasing what had been a very mild overnight.

By 9 a.m., temperatures were in the mid-50s near the lakeshore, but near 80 degrees in the far southwest suburbs of Chicagoland.

The view from the skyscrapers downtown was no less spectacular.

No, It Doesn’t Give You Pneumonia

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

A pneumonia front, first coined by the National Weather Service-Milwaukee in the 1960s, is a local term given to a back-door cold front that plunges races south over Lake Michigan in the spring and early summer.

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.

The strong density and pressure differences between the air mass behind the cold front and the air ahead of it, and the lower friction of the lake compared to land, allow these cold fronts to sweep rapidly southward down Lake Michigan and the shoreline areas, according to the National Weather Service office in Chicago. 

(MAPS: Check the Current Temperatures)

According to a 2005 study by Cory Behnke from the University of Wisconsin-Milwaukee, the magnitude of the temperature drops is strongly dependent on daytime heating and the surface water temperatures of Lake Michigan.

(MAPS: 10-Day Forecast)

This June 2018 pneumonia front was impressive, but it could have been even stronger.

As NWS-Milwaukee noted, the recent late-May heat wave sent Lake Michigan water temperatures soaring significantly above average. 

Pneumonia fronts occurring earlier in the spring, when the lake is colder, can be even more biting.

One such event in mid-April 2017 sent temperatures just after midday in Milwaukee crashing from the mid-70s to the mid-40s.

While most local residents know the “cooler by the lake” saying, this is useful to keep in mind if you’re, say, taking in a day visiting the museums in Chicago or a summer festival in Milwaukee.

2018-06-01 15:11:17

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