A rare weather system known as a "pneumonia front" is moving down from the north, dropping temperatures by 20 degrees within minutes and spawning thunderstorms.



As of 6 p.m., temperatures behind the front ranged from 56 in Milwaukee to 53 in Racine, and 65 at Northerly Island, which at 4 p.m. had still been sweltering in 84 degree-heat, according to the National Weather Service.



In the early evening, northern Cook, Kane, and DuPage counties were seeing thunderstorms, some with heavy downpours, wind gusts up to 45 mph and dime-size hail. Temperatures along the lake will drop into the 50s about an hour or so after the front passes, the weather service said. The temperature drop away from the lake will be less dramatic. Scattered showers and thunderstorms are likely to develop later this afternoon into early this evening.



A pneumonia front is a "rare meteorological phenomena" usually observed along the western shoreline of Lake Michigan and characterized by a drop of 16 degrees or more within an hour, according to the weather service. The extreme change in temperature can fuel sudden storms.



The term was first used by the Milwaukee Weather Bureau Office in the 1960s, according to WGN-TV meteorologist Tom Skilling, who said it was probably coined by the meteorologist-in-charge at the time, Reinhart Harms. He is also credited with the terms "panhandle hook" and "Alberta clipper."