An outbreak of tornadoes erupted in the Chicago area on Wednesday, causing damage in communities from Campton Hills to South Elgin. Several tornado touchdowns were reported across the area, and it appears the dangerous conditions were in part sparked by a particular weather phenomenon known as an “outflow boundary” or a “gust front.”
NBC 5 Storm Team meteorologists Brant Miller and Pete Sack explained that outflow boundaries are the leading edge of cool surface-level winds that result from the downdrafts of thunderstorms, like the ones that formed early in the afternoon on Wednesday in the Chicago area. These cool winds race toward the ground and can spread out far beyond where they originated from. When that happens, the air often bumps into warm and unstable air and then acts like a wedge, forcing that warm air upward.
New thunderstorms can often form in these areas, and if they run into the outflow boundary, the low-level wind shear can cause thunderstorms to exhibit rotation, and that’s when tornadoes can form.
As per the National Weather Service, the outflow boundaries can continue to exist for more than a day after the thunderstorms that created them disappear and can move for hundreds of miles from where they initially originated.
Chicago is no stranger to tornadoes, with many touching down in the metropolitan area over the years. The deadliest formed in Palos Hills in Cook County on April 21, 1967, killing 33 people, injuring 500, and causing more than $50 million in damage, according to the weather service. Between 1855 and 2021, the weather service recorded 97 significant tornadoes in the Chicago metro area.
On Wednesday, the FAA issued a ground stop, and passengers took shelter in tunnels at O’Hare International Airport, where more than 1,000 flights were canceled or delayed due to storms during the day. Amtrak also suspended service due to the evening storms.
The total number of tornado touchdowns from Wednesday has yet to be finalized, adding that a final number count should be completed as the week goes on. Despite the destruction, there are no reports of injuries.