A line of severe thunderstorms swept across much of northern Illinois late in the evening of June 20th, bringing corridors of typical “straight-line” (non-tornadic) wind damage along with two confirmed tornadoes in the western Chicago suburbs. Take a look at this radar image, valid shortly after 11 PM local time, revealing simultaneous tornadoes on the ground:
Radar image valid 11:09 PM CDT on June 20th, 2021. The left side of the image shows traditional “reflectivity” with the “velocity” or wind detected by the radar on the right
These tornadoes were embedded within a larger line of severe thunderstorms. While it’s more classic for tornadoes to come from individual “supercell” thunderstorms, in certain circumstances they can also develop in larger lines of storms like this one. The northern circulation was an EF-3 tornado that tracked through Naperville, Woodridge, and Burr Ridge in southern DuPage County. The southern circulation was a weaker EF-0 tornado that tracked through Plainfield in Will County.
Image of a house destroyed in Naperville, courtesy of the National Weather Service damage survey
The intensity of the tornado that tracked through DuPage County was noteworthy, with maximum winds of 140 mph making it an EF-3 tornado on the Enhanced Fujita Scale. The tornado was on the ground for a long time, from 11:02 – 11:25 PM, tracking 17.6 miles with a width of up to 600 yards. Fortunately, no deaths resulted, but at least 11 people were injured. Approximately 230 homes were damaged, with several completely destroyed or otherwise declared uninhabitable.
For reference, tornadoes that are rated EF-2 or higher are considered “strong”, with the scale maxing out at a rating of EF-5. This tornado was the first EF-2+ tornado in the Chicago metro area since June 22, 2015 when an EF-3 tornado struck Coal City (which is located in southern Will County).
Not only was it somewhat unusual (though, far from unprecedented) for such a strong tornado to come from a squall line of thunderstorms, the timing at close to midnight was also uncommon (but again, not unprecedented). The reasons these oddities occurred had to do with the environment in place for these thunderstorms. Let’s start in the upper levels of the atmosphere:
Analysis of the heights and spin in the atmosphere late in the evening on June 20th, courtesy of the Storm Prediction Center
The analysis of the set-up aloft revealed strong “forcing” across the region late in the evening of June 20th. The lines represent the “height” of a certain pressure level across the region, and note how the lines sharply decrease to the northwest of Chicago. This indicates an incoming “trough” in the jet stream. The lines fan out subtly across Wisconsin, eastern Iowa and Illinois, indicative of broad upward motion in the atmosphere. The area of color fill to the west indicates a strong area of vorticity, or spin in the atmosphere, and areas ahead of this vorticity are also in an area of upward motion. Basically, there was a lot of upward motion in the atmosphere and this contributed to severe thunderstorms maintaining themselves well into the night.
On top of this forcing, the wind shear in the atmosphere was extremely strong when the tornadoes occurred:
Analysis of the heights, temperature, and winds in the mid-levels of the atmosphere late in the evening on June 20th, courtesy of the Storm Prediction Center
Looking aloft at about 10,000 feet above the ground, winds at the time of the tornadoes were out of the west-southwest at 40-45 mph. Also, note the temperature gradient and wind shift to the west, indicative of the approaching trough and cold front (all sources of upward motion in the atmosphere). Meanwhile, surface winds where the tornadoes occurred were out of the southeast at 10-15 mph:
Surface analysis valid late in the evening of June 20th, courtesy of the Plymouth State Weather Center
This resulted in over 90 degrees of “turning” in the lowest 10,000 feet of the atmosphere, along with a 50-60 mph increase in wind speed as well. These values led to “helicity” values (a measure of wind shear) in excess of 800! For reference, values greater than 150 are considered “enough” for a tornado threat to begin.
All in all, a combination of unusually strong “forcing” and “wind shear” led to a line of thunderstorms producing two tornadoes in the middle of the night in the Chicago suburbs on June 20th, with one of these being the strongest tornado in the Chicago metro in several years.
