Drenching downpours and lots of lightning accompany heavier thunderstorms moving across sections of the Chicago metro area Tuesday, urban flood warnings issued

Tuesday’s moisture-laden atmosphere is producing clusters of strong thunderstorms which cover just 30% of the greater metro area but are hitting affected areas hard. Cook, Grundy, Kendall and Will counties have been placed under urban flood advisories because of local 2”+ rains. Rainfall has been localized but torrential under the heaviest storms Tuesday afternoon. Radar scans have indicated cloud tops up to 50,000 ft.—little surprise given the fact weather balloon soundings measured more than 1.50” is evaporated in the 70-degree dewpoint environment. This plus 90-degree temperatures prior to the onset of thunderstorms have destabilized the atmosphere and allowed thunderstorms to bubble into existence. These storms are selective—drenching one area and ignoring others. With so much available moisture, it’s little wonder our WeatherBug rain sensors indicate 1.65” has fallen at Lane Tech High School, 1.80” at southwest suburban Morris, 1.20” in Wheaton and over 1” at Wrigley Field. One National Weather Service observer reported 2.12” had fallen near Coal City in Grundy County between 2 and 2:30 pm and another thunderstorm produced 0.75” in 15 minutes at Arlington Heights. One noteworthy concern beside the periods of blinding rain and a few reports of brief wind gusts to 60 mph is the quantity of cloud to ground lightning being produced by some of these storms. Lightning data indicates as many as 1,773 cloud to ground strokes have occurred in a recent 10 minute period Tuesday afternoon. An unusually high percentage of these have been “positive” strokes, indicating the lightning discharges are lowering positive charge to earth. Studies have shown positive lightning strokes can be more energetic than the negatively charged counterparts. These often higher amperage discharges are often the most dangerous strokes, capable of setting fires and causing injury. Storm clusters are likely to affect sections of the Chicago metro area into evening then fade slowly as temperatures drop beyond sunset. New storm development is likely Wednesday afternoon and evening.

Tom Skilling
Chief Meteorologist
WGN-TV