One southwest Chicago suburb recorded rainfall so heavy this week, it typically happens just once every 50 years, according to the National Weather Service.

Much of Chicago recorded a month's worth of rainfall in 24 hours this week, with combined rain events Monday night and Tuesday afternoon totaling 3-5 inches of rain across central Cook County. Rain rates just northeast of Mokena were so torrential, they only have a 2% chance of occurring in a given year, according to National Weather Service Senior Service Hydrologist Scott Lincoln. 

This is commonly referred to as a 1-in-50 year rainfall event, but there is no guarantee another 50 years will pass before it is observed again. This week's rain follows even more rare 1,000-year and 500-year rain events observed in Cook County earlier this summer.  

Warmer air holds more moisture, so as both the global climate and Chicago area temperatures warm, heavy rain events are becoming more common and more severe. For every 1 degree Fahrenheit of warming, the air can hold 4% more moisture – and Chicago summertime temperatures have risen by 1.8 degrees Fahrenheit since 1990.

"About a century ago, a location in central Cook County had about a 1-in-4 chance of experiencing a day with 3 inches of rainfall [in a given year]," Lincoln said. "Today, it is about 1-in-2 chance."

Tuesday's storm also produced 2-3 inches of rain in less than an hour in Woodridge, Downers Grove, Westmont and Flossmoor. Many of these areas average rain events of this magnitude every 5-10 years, according to Lincoln.