The Chicago suburbs of Naperville and Woodridge, about 25 miles southwest of the city, appeared to be hardest hit. As the storm swept through this area, the Weather Service warned of “a confirmed large and extremely dangerous tornado.”
“This is a PARTICULARLY DANGEROUS SITUATION,” the Weather Service cautioned in a bulletin. “TAKE COVER NOW!”
Significant tornado damage was reported in Naperville in the vicinity of Ranchview Drive and Princeton Circle, where a “mass casualty incident” was reported due to multiple injuries and structural collapses. According to the AP, 130 damage reports came in from the suburb of 147,500 people and 22 homes were deemed uninhabitable.
The Weather Service, which surveyed the damage in Naperville, found damage consistent with an EF-3 rating on the 0 to 5 Enhanced Fujita Scale for tornado intensity. This made the twister the most intense to impact metro Chicago since 2015.
The Chicago Tribune reported 225 damaged structures in Naperville and the adjacent suburbs of Woodridge and Darien, combined.
The Weather Service in Chicago had highlighted a half-dozen areas it planned to investigate or survey for damage during the daylight hours Monday. At least 75,000 people in Illinois and adjacent Indiana remained without power Monday morning, according to PowerOutage.us.
There were no reports of fatalities associated with Sunday night’s storms.
The path of the Naperville-Woodridge tornado
Radar data suggests the tornado traveled upward of 10 miles through densely populated neighborhoods of Naperville, Woodridge, Darien and Burr Ridge. It likely first touched down near the north side of the Springbrook Prairie Forest Preserve before making its way east and crossing near the intersection of 75th Street and South Washington Street.
From there, it largely paralleled 75th Street, eventually crossing the Veterans Memorial Tollway and Interstate 55 until its ultimate demise in the vicinity of Burr Ridge.
Significant storm damage was also reported in South Haven, Ind., where the same line of storms that produced this twister may have dropped an additional tornado.
How the Naperville-Woodridge tornado developed
Severe weather had been in the forecast on Sunday, and the National Weather Service Storm Prediction Center included Chicago in a Level 3 out of 5 “enhanced risk” for severe weather. That’s where “large hail, severe/damaging winds, and a few tornadoes” were expected. It was initially unclear to what extent a tornado risk would remain present overnight as clusters of severe thunderstorms over Iowa, Missouri and western Illinois congealed into an arcing band of storms.
Doppler radar data revealed a pair of largely disorganized squall lines over northern Illinois between 9 and 10 p.m. local time. As storms approached Rochelle, the rear line weakened, allowing the leading batch of storms to become dominant. A telltale northern bookend vortex began forming south of Interstate 88 near Geneva. That marked the northern end of one segment of the squall.
About 11 p.m., a circulation gelled southwest of Naperville just south of West Ogden Avenue, going from barely noticeable to likely tornadic in one radar scan. The Weather Service quickly hoisted a tornado warning at 11:06 p.m.
By 11:09 p.m., debris was visible on radar. It eventually was lofted 15,000 to 20,000 feet high, an indicator of a strong tornado with winds at least in the EF2 range.
Radar also showed debris fallout south of downtown Chicago just before midnight local time.
The tornado was of unusual intensity for a QLCS, or “quasi-linear convect
ive system” tornado. The term is used to describe bowing squall lines with short-lived, quick-hitting circulations. Ordinarily, QLCS tornadoes form rapidly, sometimes too quickly to trigger warnings. Their circulations are often shallow and develop from the ground up, making advanced detection of cloud-based rotation virtually impossible. They ordinarily peak at EF1 strength or less.
EF3-intensity QLCS tornadoes are particularly rare. One such tornado caused multiple fatalities in El Reno, Okla., on May 25, 2019. Occurring in the dead of night, it was very narrow and on the ground for only four minutes, but it managed to toss a garbage dumpster into the second floor of a hotel.
It’s possible that the intensity of Sunday night’s tornado west of Chicago may have been the result of localized temperature contrasts stemming from early-morning cloud cover, which could have bolstered low-level spin to help strengthen any fledgling ground circulation.